CodingGuidelines: update for C99

Since 7bc341e21b (git-compat-util: add a test balloon for C99
support, 2021-12-01) we've had a hard dependency on C99, but the prose
in CodingGuidelines was written under the assumption that we were
using C89 with a few C99 features.

As the updated prose notes we'd still like to hold off on novel C99
features, but let's make it clear that we target that C version, and
then enumerate new C99 features that are safe to use.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -204,10 +204,14 @@ For C programs:
    by e.g. "echo DEVELOPER=1 >>config.mak".
 
  - We try to support a wide range of C compilers to compile Git with,
-   including old ones.  You should not use features from newer C
+   including old ones.  As of Git v2.35.0 Git requires C99 (we check
+   "__STDC_VERSION__"). You should not use features from a newer C
    standard, even if your compiler groks them.
 
-   There are a few exceptions to this guideline:
+   New C99 features have been phased in gradually, if something's new
+   in C99 but not used yet don't assume that it's safe to use, some
+   compilers we target have only partial support for it. These are
+   considered safe to use:
 
    . since early 2012 with e1327023ea, we have been using an enum
      definition whose last element is followed by a comma.  This, like