manual: Update documentation of strerror and related functions

The current implementation of strerror is thread-safe, but this
has implications for the lifetime of the return string.

Describe the strerror_l function.  Describe both variants of the
strerror_r function.  Mention the lifetime of the returned string
for strerrorname_np and strerrordesc_np.  Clarify that perror
output depends on the current locale.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>