libbinder_ndk: document env must not be null.

The current implementation does not check that env is null, even though
the documentation suggests this. Updating the documentation since if we
fix this, and apps rely on it, it could cause compatibility problems
with old devices.

Bug: N/A
Test: N/A
Change-Id: I103345afc8783be7f69b5cfd30aac90a02488f72