DOCS/contribute: stop pretending that C11 isn't allowed

Doesn't really make much of a different in practice aside from us
dropping ancient legacy fallbacks but still.

DOCS/contribute.md

@@ -140,7 +140,7 @@ Touching user-visible parts may require updating the mpv docs
 Code formatting
 ---------------
 
-mpv uses C99 with K&R formatting, with some exceptions.
+mpv uses C11 with K&R formatting, with some exceptions.
 
 - Use the K&R indent style.
 - Use 4 spaces of indentation, never use tabs (except in Makefiles).
@@ -224,15 +224,16 @@ New code must follow the following conventions:
 General coding
 --------------
 
-- Use C99. Also freely make use of C99 features if it's appropriate, such as
-  stdbool.h. (Except VLA and complex number types.)
+- Use C11. Also freely make use of C11 features if it's appropriate, but do not
+  use VLA and complex number types.
 - Don't use non-standard language (such as GNU C-only features). In some cases
   they may be warranted, if they are optional (such as attributes enabling
   printf-like format string checks). "#pragma once" is allowed as an exception.
-  But in general, standard C99 must be used.
+  But in general, standard C11 must be used.
 - The same applies to libc functions. We have to be Windows-compatible too. Use
-  functions guaranteed by C99 or POSIX only, unless your use is guarded by a
-  configure check. There is some restricted use of C11 (ask on IRC for details).
+  functions guaranteed by C11 or POSIX only, unless your use is guarded by a
+  configure check. Be mindful of MinGW-specifics since C11 support is not always
+  guaranteed.
 - Prefer fusing declaration and initialization, rather than putting declarations
   on the top of a block. Obvious data flow is more important than avoiding
   mixing declarations and statements, which is just a C90 artifact.