allow "NO NOTE" annotations, stop requiring NOTE annotations to be exhaustive

[nikomatsakis]

From #46641:

I've been very much enjoying the fact that UI tests check for ERRORs. I like being able to add minimal annotations to the test that give the kind of "overall parameters" for what the test is testing (that errors occur here, here, and here) and then having the stderr files capture the full behavior.

But there are times when I want to document a subset of the NOTEs that are present. i.e., so that I can outline "other variations in the stderr output may be acceptable, but if this NOTE disappears, that's a big problem". Note that I say subset -- often there are many notes, but it's just one piece I want to guarantee is present.

Before, we required that if you document any note, you must document all notes. The result is that it discourages me from annotating these notes that I want to be present.

(On the other hand, if you can't declare that the set of notes is exhaustive, you can't document that a note should not appear. I don't find this to be something I want to document very often, but it's certainly possible. But we could easily have a "NO_NOTE" annotation or something then.)

What I do for now is to leave comments, but I fear that future people will not read them.

cc @oli-obk @petrochenkov @estebank