CodingGuidelines: allow the use of bool

We have had a test balloon for C99's bool type since 8277dbe
(git-compat-util: convert skip_{prefix,suffix}{,_mem} to bool,
2023-12-16). As we've had it over 18 months without any complaints
let's declare it a success.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phillipwood

Documentation/CodingGuidelines

@@ -298,6 +298,9 @@ For C programs:
    . since late 2021 with 44ba10d6, we have had variables declared in
      the for loop "for (int i = 0; i < 10; i++)".
 
+   . since late 2023 with 8277dbe987 we have been using the bool type
+     from <stdbool.h>.
+
    New C99 features that we cannot use yet:
 
    . %z and %zu as a printf() argument for a size_t (the %z being for