Also document that both :deprecated and :experimental are supported
:category option values.

The locations where warnings were marked as deprecation warnings
was previously reviewed by shyouhei.

Comment a couple locations where deprecation warnings should probably
be used but are not currently used because deprecation warning
enablement has not occurred at the time they are called
(RUBY_FREE_MIN, RUBY_HEAP_MIN_SLOTS, -K).

Add assert_deprecated_warn to test assertions.  Use this to simplify
some tests, and fix failing tests after marking some warnings with
deprecated category.