[Docs] Suggest mentioning review page in the commit message

[vaivaswatha]

Summary

Most commits have a link to the corresponding reviews.llvm.org message. This isn't however mentioned in the developer policy.

[rengolin]

Actually, this is mentioned in a different document: https://www.llvm.org/docs/Phabricator.html#committing-a-change

I agree this isn't perfect and I think your change is worth doing. Can you also add a link to the URL above?

Something like "add a link to its review page, as shown in [Phab link]".

Thanks!

[vaivaswatha]

Link to Phabricator page that mentions the same topic.

[vaivaswatha]

Add missing fullstop.

[vaivaswatha]

In D98695#2628657, @rengolin wrote:

Actually, this is mentioned in a different document: https://www.llvm.org/docs/Phabricator.html#committing-a-change

I agree this isn't perfect and I think your change is worth doing. Can you also add a link to the URL above?

Something like "add a link to its review page, as shown in [Phab link]".

Thanks!

Thank you @rengolin . I've added this link now.

[lattner]

LGTM thanks!

llvm/docs/DeveloperPolicy.rst

 * Text formatting and spelling should follow the same rules as documentation
   and in-code comments, ex. capitalization, full stop, etc.
 
 * If the commit is a bug fix on top of another recently committed patch, or a
   revert or reapply of a patch, include the git commit hash of the prior
   related commit. This could be as simple as "Revert commit NNNN because it
   caused PR#".
 
+* If the patch has been reviewed, add a link to its review page, as shown
+  `here <https://www.llvm.org/docs/Phabricator.html#committing-a-change>`_.
+
 For minor violations of these recommendations, the community normally favors
 reminding the contributor of this policy over reverting. Minor corrections and
 omissions can be handled by sending a reply to the commits mailing list.
 
 Obtaining Commit Access
 -----------------------
 
 We grant commit access to contributors with a track record of submitting high