fix --from overriding annex-ignore

fix --from overriding annex-ignore

Make git-annex get/copy/move --from foo override configuration of
remote.foo.annex-ignore, as documented.

This already worked for remotes supporting hasKeyCheap. For others though,
git-annex copy --from foo would silently not do anything, while
git-annex copy --to foo would use the annex-ignored remote.

Also improved the annex-ignore docs, to reflect that `git-annex get`
without --from will skip using annex-ignored remotes, for example.

Sponsored-by: Dartmouth College's DANDI project