We are migrating Guava to use these annotations rather than jsr305's
@Nullable. We can't use the Checker Framework's _@Nullable_ yet because
we promise compatibility with Java 7, which doesn't support type
annotations. This is related to but distinct from
https://youtrack.jetbrains.com/issue/KT-21408, which is about a
different jsr305 annotation we use, @ParametersAreNonnullByDefault.
I've also updated some docs to mention Kotlin's existing support for the
Checker Framework _@NonNull_.
Chris Povirk
ac87ad422d