Rewrite pasted paragraph with historical context.

This change perhaps is somewhat controversial, but reflects honest
reality of this history of additional requirements on GPL. The
additional requirements that GPLv3 permits mostly represent historically
known situations where GPLv2 permitted license compatibility with Free
Software licenses containing such requirements.

Orthodox compatibility theory demands that such additional requirements
have explicit codification in a copyleft license, which hints at why
GPLv3 needed to include this section.

However, historical copyright holder toleration of these sorts of
requirements placed on GPLv2 works is well-documented, and failure to
mention it here is a disservice to the reader.