[3.14] gh-69011: clarify & deduplicate `ctypes.create_*_buffer` docs (GH-132858) (GH-134881)

This adds a warning about the possibly-missing NUL terminator, but in a way
that doesn't make it sound like a bug/wart.
(cherry picked from commit b783e1791b2076ef560104f8207156d75859c0a6)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>