[nn] nn.Embedding : `padding_idx` doc update (#53809)
Summary:
Follow-up of https://github.com/pytorch/pytorch/pull/53447
Reference: https://github.com/pytorch/pytorch/pull/53447#discussion_r590521051
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53809
Reviewed By: bdhirsh
Differential Revision: D27049643
Pulled By: jbschlosser
fbshipit-source-id: 623a2a254783b86391dc2b0777b688506adb4c0e
