CodingGuidelines: mention C99 features we can't use

The C99 section of the CodingGuidelines is a good overview of what we
can use, but is sorely lacking in what we can't use. Something that
comes up occasionally is the portability of %z.

Per [1] we couldn't use it for the longest time due to MSVC not
supporting it, but nowadays by requiring C99 we rely on the MSVC
version that does, but we can't use it yet because a C library that
MinGW uses doesn't support it.

1. https://lore.kernel.org/git/a67e0fd8-4a14-16c9-9b57-3430440ef93c@gmail.com/

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

Documentation/CodingGuidelines

@@ -235,6 +235,13 @@ For C programs:
    . since late 2021 with 44ba10d6, we have had variables declared in
      the for loop "for (int i = 0; i < 10; i++)".
 
+   New C99 features that we cannot use yet:
+
+   . %z and %zu as a printf() argument for a size_t (the %z being for
+     the POSIX-specific ssize_t). Instead you should use
+     printf("%"PRIuMAX, (uintmax_t)v).  These days the MSVC version we
+     rely on supports %z, but the C library used by MinGW does not.
+
  - Variables have to be declared at the beginning of the block, before
    the first statement (i.e. -Wdeclaration-after-statement).