Update documentation about copyTo and copyFrom.

Bug: 23159764
Bug: 26862970

  - Add detailed description of AutoPadding
  - Add comments to all the copy related APIs.
  - Fix typos in the comments.

Change-Id: I2d045e0d90efd94f1407f88d3e35bcd42ea93fb9
(cherry picked from commit 3231e8e0)