[ 2020-05-02; Daniel resyncs wording with recent working draft and comments ]

We have now the paper P1965R0, which introduced a specification of what friend functions in the library specification (see [hidden.friends]) are supposed to mean, there is no longer an inline definition needed to clarify the meaning. In addition to updating the change of section names the provided wording has moved the friend declaration into the public part of the class definition as have done in all other cases where we take advantage of "hidden friends" declarations.