improve doc on config.cacheJwks

- the relationship with tokenKeyUrl/discovery url already explained in
  constraints section, so no need to repeat
- add some guidance to help user decide which value to pick
- update config.tokenKeyUrl doc to help user understand that
  the token key and the JWKS are the same concept.

uaa/src/test/java/org/cloudfoundry/identity/uaa/mock/providers/IdentityProviderEndpointDocs.java

@@ -585,8 +585,8 @@ void createOAuthIdentityProvider() throws Exception {
                 fieldWithPath("originKey").required().description("A unique alias for a OAuth provider"),
                 fieldWithPath("config.authUrl").required().type(STRING).description("The OAuth 2.0 authorization endpoint URL"),
                 fieldWithPath("config.tokenUrl").required().type(STRING).description("The OAuth 2.0 token endpoint URL"),
-                fieldWithPath("config.tokenKeyUrl").optional(null).type(STRING).description("The URL of the token key endpoint which renders a verification key for validating token signatures"),
-                fieldWithPath("config.cacheJwks").optional(true).type(BOOLEAN).description("<small><mark>UAA 77.11.0</mark></small>. Option to enable or disabled the cache for the JWKS. The verification key is fetched if only `discoveryUrl` is set or `tokenKeyUrl` in combination with `tokenUrl`.").attributes(new Attributes.Attribute("constraints", "Used only if `discoveryUrl` or `tokenKeyUrl` is set.")),
+                fieldWithPath("config.tokenKeyUrl").optional(null).type(STRING).description("The URL of the token key endpoint which renders the JWKS (verification key for validating token signatures)."),
+                fieldWithPath("config.cacheJwks").optional(true).type(BOOLEAN).description("<small><mark>UAA 77.11.0</mark></small>. Option to enable caching for the JWKS (verification key for validating token signatures). Setting it to `true` increases UAA performance and is hence recommended. Setting it to `false` forces UAA to fetch the remote JWKS at each token validation, which impacts performance but may be required for when the remote JWKS changes very frequently.").attributes(new Attributes.Attribute("constraints", "Used only if `discoveryUrl` or `tokenKeyUrl` is set.")),
                 fieldWithPath("config.tokenKey").optional(null).type(STRING).description("A verification key for validating token signatures, set to null if a `tokenKeyUrl` is provided."),
                 fieldWithPath("config.userInfoUrl").optional(null).type(STRING).description("A URL for fetching user info attributes when queried with the obtained token authorization."),
                 fieldWithPath("config.showLinkText").optional(true).type(BOOLEAN).description("A flag controlling whether a link to this provider's login will be shown on the UAA login page"),
@@ -694,8 +694,8 @@ void createOidcIdentityProvider() throws Exception {
                 fieldWithPath("config.discoveryUrl").optional(null).type(STRING).description("The OpenID Connect Discovery URL, typically ends with /.well-known/openid-configurationmit "),
                 fieldWithPath("config.authUrl").optional().type(STRING).description("The OIDC 1.0 authorization endpoint URL. This can be left blank if a discovery URL is provided. If both are provided, this property overrides the discovery URL.").attributes(new Attributes.Attribute("constraints", "Required unless `discoveryUrl` is set.")),
                 fieldWithPath("config.tokenUrl").optional().type(STRING).description("The OIDC 1.0 token endpoint URL.  This can be left blank if a discovery URL is provided. If both are provided, this property overrides the discovery URL.").attributes(new Attributes.Attribute("constraints", "Required unless `discoveryUrl` is set.")),
-                fieldWithPath("config.tokenKeyUrl").optional(null).type(STRING).description("The URL of the token key endpoint which renders a verification key for validating token signatures.  This can be left blank if a discovery URL is provided. If both are provided, this property overrides the discovery URL.").attributes(new Attributes.Attribute("constraints", "Required unless `discoveryUrl` is set.")),
-                fieldWithPath("config.cacheJwks").optional(true).type(BOOLEAN).description("<small><mark>UAA 77.11.0</mark></small>. Option to enable or disabled the cache for the JWKS. The verification key is fetched if only `discoveryUrl` is set or `tokenKeyUrl` in combination with `tokenUrl`.").attributes(new Attributes.Attribute("constraints", "Used only if `discoveryUrl` or `tokenKeyUrl` is set.")),
+                fieldWithPath("config.tokenKeyUrl").optional(null).type(STRING).description("The URL of the token key endpoint which renders the JWKS (verification key for validating token signatures).  This can be left blank if a discovery URL is provided. If both are provided, this property overrides the discovery URL.").attributes(new Attributes.Attribute("constraints", "Required unless `discoveryUrl` is set.")),
+                fieldWithPath("config.cacheJwks").optional(true).type(BOOLEAN).description("<small><mark>UAA 77.11.0</mark></small>. Option to enable caching for the JWKS (verification key for validating token signatures). Setting it to `true` increases UAA performance and is hence recommended. Setting it to `false` forces UAA to fetch the remote JWKS at each token validation, which impacts performance but may be required for when the remote JWKS changes very frequently.").attributes(new Attributes.Attribute("constraints", "Used only if `discoveryUrl` or `tokenKeyUrl` is set.")),
                 fieldWithPath("config.tokenKey").optional(null).type(STRING).description("A verification key for validating token signatures. We recommend not setting this as it will not allow for key rotation.  This can be left blank if a discovery URL is provided. If both are provided, this property overrides the discovery URL.").attributes(new Attributes.Attribute("constraints", "Required unless `discoveryUrl` is set.")),
                 fieldWithPath("config.showLinkText").optional(true).type(BOOLEAN).description("A flag controlling whether a link to this provider's login will be shown on the UAA login page"),
                 fieldWithPath("config.linkText").optional(null).type(STRING).description("Text to use for the login link to the provider"),