[docs-only] added missing documentation

cs3/identity/user/v1beta1/resources.proto

@@ -30,7 +30,7 @@ option java_package = "com.cs3.identity.user.v1beta1";
 option objc_class_prefix = "CIU";
 option php_namespace = "Cs3\\Identity\\User\\V1Beta1";
 
-// A UserId represents a user.
+// A UserId represents a unique identifier of a user.
 message UserId {
   // REQUIRED.
   // The identity provider for the user.
@@ -46,14 +46,32 @@ message UserId {
 
 // Represents a user of the system.
 message User {
+  // REQUIRED.
+  // The unique identifier of this user.
   UserId id = 1;
+  // REQUIRED.
+  // A human-friendly unique identifier of this user.
   string username = 2;
+  // OPTIONAL.
+  // The e-mail address of this user.
   string mail = 3;
+  // OPTIONAL.
+  // Whether the e-mail address was verified by the IDP.
   bool mail_verified = 4;
+  // OPTIONAL.
+  // A human-friendly display name for this user, e.g. "Family and First Name"
   string display_name = 5;
+  // OPTIONAL.
+  // A list of groups this user belongs to.
   repeated string groups = 6;
+  // OPTIONAL.
+  // Opaque information.
   cs3.types.v1beta1.Opaque opaque = 7;
+  // OPTIONAL.
+  // The user id of this user in the Unix world.
   int64 uid_number = 8;
+  // OPTIONAL.
+  // The group id of this user in the Unix world.
   int64 gid_number = 9;
 }