doc: services: posix: _POSIX_THREAD_PRIO_INHERIT unsupported

_POSIX_THREAD_PRIO_INHERIT was mistakenly marked supported in
commit a07774ea presumably because we have prototypes for the
functions below.

pthread_mutexattr_getprotocol()
pthread_mutexattr_setprotocol()

However, the functions are not actually implemented in Zephyr.

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