libgui: add docs to geometry states and captureScreen

I think captureScreen is a bit fishy.  We want it to capture the
logical display, but neither of the framework or surfaceflinger is
ready.

Bug: 113041375
Test: builds
Change-Id: Id5c76a4551e5260e83e64f0ca3ed2afaa34eecb5
Merged-In: Id5c76a4551e5260e83e64f0ca3ed2afaa34eecb5
2 files changed