Bug 1402991 - Add docs for landing patches to git and mozilla-inbound

Summary:
Adds documentation to guide users when landing a revision to either
a git repository or to firefox, i.e. mozilla-inbound.

This also adds a small section to clarify the behavior of
`arc patch` which is not immediately clear unless one pays attention.
The main thing is that arc patch, by default, doesn't apply changes
to the current working set, which sort of goes against what the word
"patch" means - the behavior is fine, this doc is just to clear up
confusion.

Test Plan: Read through the changes and perhaps view it in an .rst viewer to make sure formatting is ok.

Reviewers: mcote

Reviewed By: mcote

Bug #: 1402991

Differential Revision: https://phabricator.services.mozilla.com/D95

phabricator-user.rst

@@ -324,6 +324,14 @@ this command::
 
     $ arc patch <revision id>
 
+It is helpful to understand that ``arc patch``, by default, will not attempt to
+patch the revision on top of your current working set. Instead, it applies the
+changes on top of the same parent commit the author used and creates a new
+commit and a new branch (git) or bookmark (hg). If it cannot find the same
+parent commit in your local repo then it will warn you and give you the option
+to apply it on top of the current working set. If you wish to test a revision
+on top of your current working set use ``arc patch --nobranch``.
+
 If you have a stack of revisions (see above section
 :ref:`series-of-commits`), the commits from all previous revisions
 will be applied as well.  Note that if you are pulling down a stack of
@@ -374,6 +382,24 @@ and to communicate what they should do next.  These actions include:
   ideally a substitute reviewer or other action for the author to
   take, if there are no longer sufficient reviewers on the revision.
 
+***************
+Landing Patches
+***************
+
+Landing to git repositories is simple: ``arc land --revision D123``. arc
+will apply the revision on top of the master branch and push to the remote
+origin, assuming you have permission. You can override the "master" branch with
+the ``--onto`` flag and you can override the remote origin with the ``--remote``
+flag.
+
+Manually landing to mozilla-inbound is also straightforward: check out the
+``inbound`` bookmark, run ``arc patch D123 --nobranch``, and then push the
+commits to mozilla-inbound as normal. The ``--nobranch`` flag is needed because
+otherwise arc will create a new commit and bookmark that is not based on the tip
+of ``inbound``. If the command fails, run ``arc patch D123`` and then manually
+rebase the commits on top of the ``inbound`` bookmark.
+
+
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