Suppress auto-doc when "null" manually documented.

When API authors explicitly call out "null" related behavior in
their manual docs, it's jarring to reproduce that information with
the new @Nullable and @NonNull auto-doc feature.

So detect APIs that manually talk about their "null" behavior and
suppress auto-doc in those cases.

Test: make -j32 offline-sdk-docs
Bug: 37526420
Change-Id: I3b741b5e5b6cffa10a85a64b8008d8c1befecc3c