doc: posix: add links and make minor corrections

Add a link to _POSIX_CLOCK_MONOTONIC, correct the
_POSIX_RAW_SOCKETS kconfig option.

Define _POSIX_CHOWN_RESTRICTED and _POSIX_NO_TRUNC so that they
conform to the spec.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>