sharedmem.h: fix the docs.

The complaint was that the docs were vague. Unfortunately, the truth is
a bit of a mess, but that's all the more reason for us to document it
clearly.

Bug: https://github.com/android/ndk/issues/1127
Test: N/A
Change-Id: I3ba116d43bb99731e0bb0832118e0d58a6400ad0