docs: posix: update the XSI_THREADS_EXT option group

Previously pthread_attr_getguardsize() and
pthread_attr_setguardsize() were part of the
XSI_THREADS_EXT option group. That is no longer the case.

In IEEE 1003.1-2017 they are part of the POSIX_THREADS_EXT
option group.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>
1 file changed