Be clearer about linker warnings.

Explicitly say "warning" for warnings, explicitly say what action
we're going to take (such as "(ignoring)"), always provide a link to
our documentation when there is one, explicitly say what API level the
behavior changes at, and explicitly say why we're allowing the misbehavior
for now.

Bug: http://b/71852862
Test: ran tests, looked at logcat
Change-Id: I1795a5af45deb904332b866d7d666690dae4340b