[docs][Phabricator] git migration related update

[jsji]

Summary

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

[probinson]

Content looks fine, one style note.

llvm/docs/Phabricator.rst

198

Don't start a sentence with "eg." Instead use "For example, ensure ..."

[jsji]

Address comments

[probinson]

LGTM

llvm/docs/Phabricator.rst

 Note that if you use the Arcanist tool the ``Differential Revision`` line will
 be added automatically. If you don't want to use Arcanist, you can add the
 ``Differential Revision`` line (as the last line) to the commit message
 yourself.
 
 Using the Arcanist tool can simplify the process of committing reviewed code as
 it will retrieve reviewers, the ``Differential Revision``, etc from the review
 and place it in the commit message. You may also commit an accepted change
-directly using ``git llvm push``, per the section in the :ref:`getting started
+directly using ``git push``, per the section in the :ref:`getting started
 guide <commit_from_git>`.
 
 Note that if you commit the change without using Arcanist and forget to add the
 ``Differential Revision`` line to your commit message then it is recommended
 that you close the review manually. In the web UI, under "Leap Into Action" put
 the git revision number in the Comment, set the Action to "Close Revision" and
 click Submit.  Note the review must have been Accepted first.
 
 
 Committing someone's change from Phabricator
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 On a clean Git repository on an up to date ``master`` branch run the
 following (where ``<Revision>`` is the Phabricator review number):
 
 ::
 
   arc patch D<Revision>
 
 
 This will create a new branch called ``arcpatch-D<Revision>`` based on the
 current ``master`` and will create a commit corresponding to ``D<Revision>`` with a
 commit message derived from information in the Phabricator review.
 
-Check you are happy with the commit message and amend it if necessary. Then,
-make sure the commit is up-to-date, and commit it. This can be done by running
+Check you are happy with the commit message and amend it if necessary.
+eg:Ensure the 'Author' property of the commit is set to the original author.
+You can use a command to correct the author property if it is incorrect:
+
+::
+
+  git commit --amend --author="John Doe <[e-mail]>"
+
+Then, make sure the commit is up-to-date, and commit it. This can be done by running
 the following:
 
 ::
 
   git pull --rebase origin master
   git show # Ensure the patch looks correct.
   ninja check-$whatever # Rerun the appropriate tests if needed.
-  git llvm push
+  git push
 
 Or
 
 ::
 
   arc land D<Revision>