Add descriptions to compat changes.

Capture the comment above a definition of a compat change and make it
the description.
Next: make sure existing changes use supported format (only /**
comments, only above the annotations), and use in developer UI.

Bug: 144927670
Test: treeHugger
Change-Id: I5f53517751762a637dc6fc0f220b83d84eb3a916
1 file changed