--strict flags

TL;DR restore the meaning of --static

elaborating the parenthesis in my pull request (#20982) ...

i was not serious when i suggested adding an --all flag. i prefer having --strict enable more flags, and the documentation supports this change (#3c9ee34). more precisely, i think --strict should enable two more flags:

• --warn-unreachable is not a type checking feature, but i think the bugs it finds should be fixed. they are easy to fix, and enabling this flag would be consistent with the documentation: "This subset primarily includes checks for inadvertent type unsoundness." [emphasis added]

• many people will expect --strict to enable --strict-equality-for-none. the bugs found by the new check are easy to fix, and they are also in my should-fix category. (trailing whitespace might be the best example of a should-fix bug.)

further simplification is possible. having --strict-equality-for-none imply --strict-equality would reduce command line length when the new check is desired. when i enabled this check, it found no errors in my code. so i think removing the flag and extending --strict-equality deserves consideration.

Start with docs/source/command_line.rst and the discussion in pull request #20982. Review how the cited strict-related flags are currently defined and decide which should be enabled or implied; done means an agreed flag policy reflected consistently in the implementation and documentation.