update the phabricator docs to reflect the monorepo

[froydnj]

Summary

There's only one right choice for the repository field, and a lot of
wrong ones.

[froydnj]

Not sure who needs to review this; picking a familiar face. ;)

[echristo]

LGTM! Thanks!

-eric

[froydnj]

Cool, thank you! Could you also land this for me? I do not have commit privileges. Thanks!

llvm/docs/Phabricator.rst

 * Leave this first Repository field blank. (We'll fill in the Repository
   later, when sending the review.)
 * Leave the drop down on *Create a new Revision...* and click *Continue*.
 * Enter a descriptive title and summary.  The title and summary are usually
   in the form of a :ref:`commit message <commit messages>`.
 * Add reviewers (see below for advice). (If you set the Repository field
   correctly, llvm-commits or cfe-commits will be subscribed automatically;
   otherwise, you will have to manually subscribe them.)
-* In the Repository field, enter the name of the project (LLVM, Clang,
-  etc.) to which the review should be sent.
+* In the Repository field, enter "rG LLVM Github Monorepo".
 * Click *Save*.
 
 To submit an updated patch:
 
 * Click *Differential*.
 * Click *+ Create Diff*.
 * Paste the updated diff or browse to the updated patch file. Click *Create Diff*.
 * Select the review you want to from the *Attach To* dropdown and click