Merge branch 'jc/post-c89-rules-doc' into maint

We have been trying out a few language features outside c89; the
coding guidelines document did not talk about them and instead had
a blanket ban against them.

* jc/post-c89-rules-doc:
CodingGuidelines: spell out post-C89 rules

diff --cc Documentation/CodingGuidelines
Simple merge