perlxs: Add text about dealing with locales, etc

This pod has been totally silent about monkey wrenches that could be
thrown by XS code doing things that can affect perl.  Add a few
cautions, including some detailed information about one area where we
have been bitten: locales.

(cherry picked from commit 8a384d3a99d847516ae513b3b71f778d16251471)
perlapi: Add docs for sync_locale()
perldelta: Add entry for sync_locale()

(cherry picked from commit d8bcb4d6a785b98e8fa0eeaf863b9b4a64fcad35)
perldelta for 692c22c646
Finalize perldelta except for acknowledgements
Add new release to perlhist

(cherry picked from commit 36719686e209d3a2120ac01da5207598078e07a7)