Document support for @NullMarked / @NullUnmarked once JSpecify 0.3.0 is out

[lazaroclapp]

NullAway after 0.10.0 has some support for both @NullMarked and @NullUnmarked (see JSpecify docs here).

We support both annotations on: package, class, and method scopes.

However, two reasons keep us from updating the docs advertising this currently:

• JSpecify 0.2.0 doesn't yet have @NullUnmarked, we are basically tracking JSpecify main/0.3.0 here and should wait for it to be released before finalizing anything about our support for these two annotations.
• We don't support the annotation at a module level, because we still maintain JDK 8 compatibility. We might want to add that after Remove support for running on JDK / Java 8 #634 .