[PR] Add documentation for $ObjMapi<T, F>
[PR] Add documentation for $ObjMapi<T, F>

Summary:
Ref #2464

This PR documents the $ObjMapi utility type.
Pull Request resolved: https://github.com/facebook/flow/pull/5576

Reviewed By: gabelevi

Differential Revision: D14555190

Pulled By: panagosg7

fbshipit-source-id: 64891ca6486cc2ffea2d3fc96d1902a6a7f8ec4e

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow