Camera2: Update lensShadingMap documentation

Revert added language about shading map being the full flat-field
correction; it's actually the same correction as applied to camera
device-processed data.

Also fix a few other wording issues in the lens shading documentation.

Bug: 18175853
Change-Id: I27691925e6496afe18f3506084d89f2523b5555d
1 file changed