Fixed required fields and paths list (#39358) (#39428)

Some small fix for the `x-pack` rest api spec.

* In both `security.enable_user.json` and `security.disable_user.json`
   the `username` parameter was `false` instead of `true`
   (the documentation is already correct).
* In `security.get_privileges.json` there were missing all the
   possible paths since the path parameters are not required.
   This fix aligns the document with the rest of the spec,
   where all the possible combinations are listed.

x-pack/plugin/src/test/resources/rest-api-spec/api/security.disable_user.json

@@ -11,7 +11,7 @@
         "username": {
           "type" : "string",
           "description" : "The username of the user to disable",
-          "required" : false
+          "required" : true
         }
       },
       "params": {

x-pack/plugin/src/test/resources/rest-api-spec/api/security.enable_user.json

@@ -11,7 +11,7 @@
         "username": {
           "type" : "string",
           "description" : "The username of the user to enable",
-          "required" : false
+          "required" : true
         }
       },
       "params": {

x-pack/plugin/src/test/resources/rest-api-spec/api/security.get_privileges.json

@@ -5,6 +5,8 @@
     "url": {
       "path": "/_security/privilege/{application}/{name}",
       "paths": [
+        "/_security/privilege",
+        "/_security/privilege/{application}",
         "/_security/privilege/{application}/{name}"
       ],
       "parts": {