[mdoc] Fixes an update issue with generic and non-generic members.

[mdoc] Fixes an update issue with generic and non-generic members.
The existing update was not doing a good job of matching members that have both generic and non-generic members. So for example, if you had a method, `Foo<T>`, and added a non-generic version in a future update (`Foo`); you could end up with a situation where one of the members was being duplicated.