improve documentation about OpenId connect issuer configuration

Signed-off-by: Thomas Jaeckle <thomas.jaeckle@bosch.io>

documentation/src/main/resources/pages/ditto/installation-operating.md

@@ -90,7 +90,7 @@ proxy_set_header              x-ditto-pre-authenticated "nginx:${remote_user}";
 
 The authentication provider must be added to the ditto-gateway configuration.
 `auth-subjects`, an optional field, takes a list of placeholders that will be
-evaluated against incoming JWT's.
+evaluated against incoming JWTs.
 For each entry in `auth-subjects` and authorization subject will be generated.
 If the entry contains unresolvable placeholders, it will be ignored in full.
 When `auth-subjects` is not provided, the “sub” claim (`{%raw%}{{ jwt:sub }}{%endraw%}`) is used by default.
@@ -115,6 +115,9 @@ ditto.gateway.authentication {
 }
 ```
 
+{% include note.html content="The issuer **must not** include the `http://` or `https://` prefix as this is added 
+    based on the configuration value of `ditto.gateway.authentication.oauth.protocol`." %}
+
 In order to do this by specifying a Java system property, use the following:
 
 ```shell