message: Documentation for read_concern::acknowledge_string() should clarify that any read concern string can be passed in
diff: @@ -121,9 +121,10 @@ class MONGOCXX_API read_concern {
  level acknowledge_level() const;
  ///
  - /// Sets the read concern string.
  - ///
  - /// One of "local", "majority", "linearizable", or "".
  + /// Sets the read concern string. Any valid read concern string (e.g. "local",
  + /// "majority", "linearizable", "") may be passed in. For forward-compatibility
  + /// with read concern levels introduced in the future, no validation is performed on
  + /// this string.
  ///
  /// @param rc_string
  /// The read concern string.
  