Improve ALooper_pollAll/ALooper_pollOnce docs.

Add the soon-to-be-required removal message for ALooper_pollAll, and
expand the docs for both ALooper_pollAll and ALooper_pollOnce to make
the reason for removal clear, and to explain the fix callers should
apply.

Bug: https://github.com/android/ndk/issues/2014
Test: treehugger
Change-Id: I020a280346d1ff9f5fcbec4c153dee8319cdf539