Fix comments/pod for LC_NUMERIC not always C

Fix comments/pod for LC_NUMERIC not always C

In recent Perl versions, the underlying locale for LC_NUMERIC has been
kept in C because XS code is expecting a dot radix character.  But if
the LC_NUMERIC locale has a dot, that is unnecessary.  (There is also
the thousands grouping separator which for safety we verify is empty.)
Thus 5.27 doesn't always keep the underlying locale in C; it does so
only if necessary.

This commit updates various comments and pods to reflect this change.