Camera2 NDK: Update lensShadingMap documentation, remove duplicate

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.

Remove the duplicate lens shading map entry in the metadata tables.

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

Bug: 18175853
Change-Id: I007aaf8b4901729afb116331d19926a6c2eea60a