bpo-1294959: Try to clarify the meaning of platlibdir (GH-20332)

bpo-1294959: Try to clarify the meaning of platlibdir (GH-20332)

Try to make the meaning of platlibdir clear.  The previous wording could
be misinterpreted to suggest that it will be used to find all shared
libraries on the system, and not just Python extensions.  Furthermore,
it was unclear whether it affects third-party (site-packages) extensions
or not.  The new wording tries to make its dual purpose clear,
and provide the additional example of extensions in site-packages.
(cherry picked from commit 242d95659b6b4ff4fb54b58a30454dafa311d4e9)

Co-authored-by: Michał Górny <mgorny@gentoo.org>