doc: Mention a wctype bug

[Bruno Haible]

The 'wctype' implementation on mingw does not support the "blank" property,
as evidenced by this program:

================================================================================
#include <stdio.h>
#include <wchar.h>
#include <wctype.h>
#include <ctype.h>
int main ()
{
  printf ("%d\n", !!isblank (' '));
  printf ("%d\n", !!iswblank ((wchar_t)' '));
  printf ("%d\n", !!iswctype ((wchar_t)' ', wctype ("blank")));
  printf ("%lu %lu\n", (unsigned long) wctype("space"), (unsigned long) 
wctype("blank"));
}
================================================================================
which prints:
1
1
0
8 0


2023-07-22  Bruno Haible  <bruno@clisp.org>

        doc: Mention a wctype bug.
        * doc/posix-functions/wctype.texi: Mention a bug on mingw.

diff --git a/doc/posix-functions/wctype.texi b/doc/posix-functions/wctype.texi
index 4d1deeb904..a158cdd102 100644
--- a/doc/posix-functions/wctype.texi
+++ b/doc/posix-functions/wctype.texi
@@ -25,4 +25,7 @@
 However, the Gnulib function @code{c32_get_type_test}, provided by Gnulib
 module @code{c32_get_type_test}, operates on 32-bit wide characters and
 therefore does not have this limitation.
+@item
+This function does not support the @code{"blank"} argument on some platforms:
+mingw.
 @end itemize