LC_NUMERIC documentation updates + tests

LC_NUMERIC documentation updates + tests

Most of the confusion around LC_NUMERIC was fixed with commits
7e4353e96785be675a69a6886d154405dbfdc124 and
2095dafae09cfface71d4202b3188926ea0ccc1c
but two errors remain:

- the early parts of perllocale.pod still say printf() uses LC_NUMERIC
  with just 'use locale' when actually a POSIX::setlocale() call is
  also needed

- format() hasn't used LC_NUMERIC unconditionally since 5.005_03
  (commit 097ee67dff1c60f201bc09435bc6eaeeafcd8123).

Update the documentation and test the claims in t/run/locale.t.