doc: posix: correct note of XSI_THREADS_EXT

Previously, XSI_THREADS_EXT included a note about
undefined behaviour. However, the required functions
pthread_attr_getstack(), pthread_attr_setstack(),
pthread_getconcurrency(), and pthread_setconcurrency()
are all conformant.

Remove the unneeded note.

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