Add/edit docs for nn.transformer (#21746)
Summary:
Add docs for TransformerEncoder and TransformerDecoder, plus minor edits.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21746
Differential Revision: D15807498
Pulled By: zhangguanheng66
fbshipit-source-id: 388efb5821c4c3d25865cecea70902e9b2bf5d15
