doc: Mention a macOS bug

[Bruno Haible]

Some unit test of the pthread_rwlock_trywrlock function, that I added to
the maint-tools repository, exhibits a problem on macOS 12. Documented like
this:


2024-08-06  Bruno Haible  <bruno@clisp.org>

        doc: Mention a macOS bug.
        * doc/posix-functions/pthread_rwlock_trywrlock.texi: Mention a macOS
        bug.

diff --git a/doc/posix-functions/pthread_rwlock_trywrlock.texi 
b/doc/posix-functions/pthread_rwlock_trywrlock.texi
index fc20316c5e..75367497da 100644
--- a/doc/posix-functions/pthread_rwlock_trywrlock.texi
+++ b/doc/posix-functions/pthread_rwlock_trywrlock.texi
@@ -17,4 +17,9 @@
 
 Portability problems not fixed by Gnulib:
 @itemize
+@item
+This function returns @code{EDEADLK} instead of @code{EBUSY}
+when the current thread already owns the lock for writing
+on some platforms:
+macOS 12.
 @end itemize