Document that arc diff is able to commit

Test Plan: Read it.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2025

Differential Revision: https://secure.phabricator.com/D4008

src/docs/userguide/arcanist_diff.diviner

@@ -56,10 +56,13 @@ That's a fancy way of saying "all the commits on the current branch that
 you haven't pushed yet". So, to **create a revision** in Git, run:
 
   $ nano source_code.c  # Make changes.
-  $ git commit          # Commit changes.
+  $ git commit -a       # Commit changes.
   $ arc diff            # Creates a new revision out of ALL unpushed commits on
                         # this branch.
 
+The `git commit` step is optional. If there are uncommitted changes in the
+working copy then Arcanist will ask you to create a commit from them.
+
 Since it uses **all** the commits on the branch, you can make several commits
 before sending your changes for review if you prefer.
 
@@ -77,9 +80,12 @@ if you aren't creating branches-on-branches, juggling remotes, etc.
 To **update a revision**, just do the same thing:
 
   $ nano source_code.c  # Make more changes.
-  $ git commit          # Commit them.
+  $ git commit -a       # Commit them.
   $ arc diff            # This prompts you to update revision information.
 
+The `git commit` step is optional. If there are uncommitted changes in the
+working copy then Arcanist will ask you to amend them to the commit.
+
 When your revision has been accepted, you can usually push it like this:
 
   $ arc land <branch>   # Merges <branch> into master and pushes.
@@ -103,6 +109,9 @@ To **create a revision** in Mercurial, run:
   $ arc diff            # Creates a new revision out of ALL outgoing commits
                         # on this branch since the last merge.
 
+The `hg commit` step is optional. If there are uncommitted changes in the
+working copy then Arcanist will ask you to create a commit from them.
+
 Since it uses **all** the outgoing commits on the branch, you can make several
 commits before sending your changes for review if you prefer.
 
@@ -119,6 +128,10 @@ To **update a revision**, just do the same thing:
   $ hg commit           # Commit changes.
   $ arc diff            # This prompts you to update revision information.
 
+The `hg commit` step is optional. If there are uncommitted changes in the
+working copy then Arcanist will ask you to create a commit from them (or amend
+them to the previous commit if supported).
+
 When your revision has been accepted, push it normally. (`arc` does not have
 push integration in Mercurial because it can't force merges and thus can't
 guarantee it will be able to do anything useful.)

src/docs/userguide/arcanist_quick_start.diviner

@@ -72,23 +72,9 @@ to your Phabricator account.
 For detailed instructions on using `arc diff`, see
 @{article:Arcanist User Guide: arc diff}.
 
-Subversion:
-
   $ $EDITOR file.c
   $ arc diff
 
-Git:
-
-  $ $EDITOR file.c
-  $ git commit -a -m '...'
-  $ arc diff HEAD^
-
-Mercurial:
-
-  $ $EDITOR file.c
-  $ hg commit -m '...'
-  $ arc diff tip^
-
 = Next Steps =
 
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