[quant] add docs for embedding/embedding_bag (#51770)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/51770
Test Plan:
tested locally on mac
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D26279112
fbshipit-source-id: 8675d3ef712ecbe545bad0d3502181b3ccdd7f89
