Rollup merge of #145324 - Kobzol:bootstrap-host-only, r=jieyouxu

Rename and document `ONLY_HOSTS` in bootstrap

Everytime I examined the `ONLY_HOSTS` flag of bootstrap steps, I was utterly confused. Why is it called ONLY_HOSTS? How does the fact that it is skipped if `--target` is passed, but `--host` is not (which was not accurate) help me?

The reality of the flag is that if it is true, the targets for which the given Step will be built is determined based on the `--host` flag, while if it is false, it is determined based on the `--target` flag, that's pretty much it. The previous comment was just a (not very helpful and not even accurate) corollary of that.

I clarified the comment, and also renamed the flag to `IS_HOST` (happy to brainstorm better names, but the doc. comment change is IMO the main improvement).

r? ``@jieyouxu``

Author: Kobzol