Add `short definition`, which is more useful to human readers than `signature`.
Add `short definition`, which is more useful to human readers than `signature`.

Summary:
For example,

`signature`: `addPlan(Plan)`
`short definition`: `void addPlan(Plan)`

`signature`: `FULL_DURATION`
`short definition`: `long FULL_DURATION`

Reviewers: O2 Material Motion, featherless

Reviewed By: O2 Material Motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D1424

material-motion/apidiff,material-motion/apidiff,material-motion/apidiff