update F.grid_sample doc for clarity (#19754)
Summary:
https://github.com/pytorch/pytorch/issues/19717
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19754
Differential Revision: D15085449
Pulled By: soumith
fbshipit-source-id: 0dda05bd395d58a496bf397ca7f1c50a239b0ed1
