[docs][Phabricator] git migration related update

1.Add instructions to update author when committing other's patch

We have updated DeveloperPolicy to show how to change author in
https://reviews.llvm.org/D72468

We should also update Phabricator page to include such infomation,
in case people follow the steps here and forget to update author info.

2. Replace `git llvm push` with `git push`

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D76718