Add reference to _user_mail_text() in the doxygen for user_mail_tokens() function.

The user_mail_tokens() function doesn't seem to be called anywhere in core.

The only inline documentation is a reference to user_mail() which gives no clue.

The d6 version of user_mail_tokens had a completely different function signature, so the d7 version obviously doesn't exist for the sake of backward compatibility.

Does anything in core call this function? Or is it cruft that should be deleted?