input.h: add missing __INTRODUCED_IN and doc comments.

These were missing from the original commit, and are causing developer
confusion.

Bug: https://github.com/android/ndk/issues/1515
Bug: https://issuetracker.google.com/116830907
Test: treehugger
Change-Id: I4324a0ec9be2b5ce334994b27cd4069d63060d1d
1 file changed