KAFKA-3097: Update docs to mention PrincipalType "User" is case sensi…

…tive (apache#5734)

Reviewers: Jun Rao <junrao@gmail.com>

docs/security.html

@@ -1018,7 +1018,7 @@ <h3><a id="security_authz" href="#security_authz">7.4 Authorization and ACLs</a>
     <pre>authorizer.class.name=kafka.security.auth.SimpleAclAuthorizer</pre>
     Kafka acls are defined in the general format of "Principal P is [Allowed/Denied] Operation O From Host H on any Resource R matching ResourcePattern RP". You can read more about the acl structure in KIP-11 and resource patterns in KIP-290. In order to add, remove or list acls you can use the Kafka authorizer CLI. By default, if no ResourcePatterns match a specific Resource R, then R has no associated acls, and therefore no one other than super users is allowed to access R. If you want to change that behavior, you can include the following in server.properties.
     <pre>allow.everyone.if.no.acl.found=true</pre>
-    One can also add super users in server.properties like the following (note that the delimiter is semicolon since SSL user names may contain comma).
+    One can also add super users in server.properties like the following (note that the delimiter is semicolon since SSL user names may contain comma). Default PrincipalType string "User" is case sensitive.
     <pre>super.users=User:Bob;User:Alice</pre>
     By default, the SSL user name will be of the form "CN=writeuser,OU=Unknown,O=Unknown,L=Unknown,ST=Unknown,C=Unknown". One can change that by setting a customized PrincipalBuilder in server.properties like the following.
     <pre>principal.builder.class=CustomizedPrincipalBuilderClass</pre>
@@ -1119,19 +1119,19 @@ <h4><a id="security_authz_cli" href="#security_authz_cli">Command Line Interface
         </tr>
         <tr>
             <td>--allow-principal</td>
-            <td>Principal is in PrincipalType:name format that will be added to ACL with Allow permission. <br>You can specify multiple --allow-principal in a single command.</td>
+            <td>Principal is in PrincipalType:name format that will be added to ACL with Allow permission. Default PrincipalType string "User" is case sensitive. <br>You can specify multiple --allow-principal in a single command.</td>
             <td></td>
             <td>Principal</td>
         </tr>
         <tr>
             <td>--deny-principal</td>
-            <td>Principal is in PrincipalType:name format that will be added to ACL with Deny permission. <br>You can specify multiple --deny-principal in a single command.</td>
+            <td>Principal is in PrincipalType:name format that will be added to ACL with Deny permission. Default PrincipalType string "User" is case sensitive. <br>You can specify multiple --deny-principal in a single command.</td>
             <td></td>
             <td>Principal</td>
         </tr>
         <tr>
             <td>--principal</td>
-            <td>Principal is in PrincipalType:name format that will be used along with --list option. This will list the ACLs for the specified principal. <br>You can specify multiple --principal in a single command.</td>
+            <td>Principal is in PrincipalType:name format that will be used along with --list option. Default PrincipalType string "User" is case sensitive. This will list the ACLs for the specified principal. <br>You can specify multiple --principal in a single command.</td>
             <td></td>
             <td>Principal</td>
         </tr>