improve documentation for sampler (#338)
improve documentation for sampler (#338)

Summary:
(fix https://github.com/facebookresearch/detectron2/issues/431)
Pull Request resolved: https://github.com/fairinternal/detectron2/pull/338

Differential Revision: D18754819

Pulled By: ppwwyyxx

fbshipit-source-id: e30eef65717a7dadc7f9dc3c8b7e93b592959092

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2