Fix documentation for User.display_name

[IndorilReborn]

Summary

The User.display_name docstring is a copy of Member.display_name one, but it does not actually try to fetch a guild-specific display_name
https://github.com/Rapptz/discord.py/blob/master/discord/user.py#L313

    @property
    def display_name(self) -> str:
        """:class:`str`: Returns the user's display name.

        For regular users this is just their global name or their username,
        but if they have a guild specific nickname then that
        is returned instead.
        """
        if self.global_name:
            return self.global_name
        return self.name

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

Not quite.

This is the BaseUser docstring which applies to all the BaseUser subclasses. Some of these are also Member-like (e.g. WidgetMember) so the docstring is supposed to be correct there. That being said, it seems WidgetMember's implementation is bugged anyhow.