update documentation of PairwiseDistance#19241 (#19412)
Summary:
Fix the documentation of PairwiseDistance [#19241](https://github.com/pytorch/pytorch/issues/19241)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19412
Differential Revision: D14998271
Pulled By: soumith
fbshipit-source-id: bcb2aa46d3b3102c4480f2d24072a5e14b049888
