[docs] Update Phabricator docs about setting repository for diffs uploaded via web

[benhamilton]

Summary

Docs are out of date now that we have separate repositories for LLVM,
Clang, etc.

[asb]

Am I correct in thinking that as long as the repository field is filled out, there should never be a reason to explicitly add llvm-commits or cfe-commits to the subscriber list? If so it would probably make sense to remove the note "subscribe mailing lists that you want to be included in the review. If your patch is for LLVM, add llvm-commits as a Subscriber; if your patch is for Clang, add cfe-commits." Instead, you could expand the bullet point on the repository field to say "Entering the correct repository will ensure that the appropriate mailing list (llvm-commits or cfe-commits) is automatically subscribed to patch patch updates.

[benhamilton]

• Update info on subscribing llvm-commits/cfe-commits

[benhamilton]

there should never be a reason to explicitly add llvm-commits or cfe-commits to the subscriber list?

Good catch, fixed.

[asb]

Looks good to me!

docs/Phabricator.rst

 * ``git format-patch -U999999 @{u}``
 * ``svn diff --diff-cmd=diff -x -U999999``
 
 To upload a new patch:
 
 * Click *Differential*.
 * Click *+ Create Diff*.
 * Paste the text diff or browse to the patch file. Click *Create Diff*.
-* Leave the Repository field blank.
+* In the Repository field, enter the name of the project (LLVM, Clang,
+  etc.) to which the review should be sent.
 * 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) and subscribe mailing
   lists that you want to be included in the review. If your patch is
   for LLVM, add llvm-commits as a Subscriber; if your patch is for Clang,
   add cfe-commits.
 * Leave the Repository and Project fields blank.