bpo-22257: Mention startup refactoring in What's New (GH-4286)

bpo-22257: Mention startup refactoring in What's New (GH-4286)

While technically a purely internal change, bpo-31845 was
a fairly significant externally visible bug caused by
these changes (environment variable based configuration
was being ignored due to a change in the relative order
of reading the environment and reading command line settings,
and the test suite was only testing the command line options)

Hence this note to essentially say "If you see odd startup
problems in 3.7 that you've never seen in previous releases,
it's probably our fault, so let us know, and we'll fix it".