perlhacktips: document our usage of C99 bool

Since commit b1c011d we unconditionally include <stdbool.h> and
assume C99 booleans are available, so document the feature in
perlhacktips.

pod/perlhacktips.pod

@@ -128,8 +128,7 @@ everywhere.  It's fine to probe for additional C99 features and use
 them where available, providing there is also a fallback for compilers
 that don't support the feature.  For example, we use C11 thread local
 storage when available, but fall back to POSIX thread specific APIs
-otherwise, and we use C<char> for booleans if C<< <stdbool.h> >> isn't
-available.
+otherwise.
 
 Code can use (and rely on) the following C99 features being present
 
@@ -233,6 +232,13 @@ C<//> comments
 All compilers we tested support their use. Not all humans we tested
 support their use.
 
+=item *
+
+booleans
+
+You can use C<bool>, C<true>, and C<false> as provided by C<< <stdbool.h >>
+(or natively in C++).
+
 =back
 
 Code explicitly should not use any other C99 features. For example