Updating doxygen documentation.

Added mechanism to document API changes in the generated doxygen
documentation. Developers are now expected to keep MajorAPIChanges.md
file updated as they change APIs or major design features.

This change now requires doxygen 1.8 or greater since we now rely on
markdown support in doxygen.

Change-Id: I55b340625cbd21cff7913d1607062c0cb4d72465
Merge topic 'cleanup_doxygen' into next

4fdd45b1 Updating doxygen documentation.
More fixes for doxygen.

Change-Id: I4aaddcc125ec77288cc04c87c2559c8f4c253255
Merge topic 'cleanup_doxygen' into next

49dd4f17 More fixes for doxygen.
Updating doxygen documentation.

Added mechanism to document API changes in the generated doxygen
documentation. Developers are now expected to keep MajorAPIChanges.md
file updated as they change APIs or major design features.

This change now requires doxygen 1.8 or greater since we now rely on
markdown support in doxygen.

Change-Id: I55b340625cbd21cff7913d1607062c0cb4d72465
Merge topic 'cleanup_doxygen' into next

404d9b79 Updating doxygen documentation.