docs: clarify smooth l1 == l1 when beta == 0 (#70673)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/68558.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70673
Reviewed By: albanD
Differential Revision: D33430267
Pulled By: jbschlosser
fbshipit-source-id: db92187ff4f2799b19a6c4a5a6b653e9211c3aca
