Docs: add more detail to SurfaceTexture#getTimestamp

Copies language from <android/surface_texture.h> that connects
EGL/Vulkan presentation timestamps to SurfaceTexture#getTimestamp.

In the process, reorganized the existing language to talk about
general properties of the timestamp, and then talk about behavior of
specific producers instead of mixing them together.

Bug: 72755997
Test: make docs
Change-Id: I88253f9e23df7d7c4fe3c1c51ff2ac494da1913c