Document rebase background used in Differential

Summary:
Also fixed formatting.
Also linked docs.

Test Plan:
  $ diviner .

Reviewers: epriestley, wez, nh

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3678

src/applications/audit/application/PhabricatorApplicationAudit.php

@@ -30,6 +30,10 @@ final class PhabricatorApplicationAudit extends PhabricatorApplication {
     return 'audit';
   }
 
+  public function getHelpURI() {
+    return PhabricatorEnv::getDoclink('article/Audit_User_Guide.html');
+  }
+
   public function getRoutes() {
     return array(
       '/audit/' => array(

src/applications/differential/application/PhabricatorApplicationDifferential.php

@@ -30,6 +30,10 @@ final class PhabricatorApplicationDifferential extends PhabricatorApplication {
     return 'differential';
   }
 
+  public function getHelpURI() {
+    return PhabricatorEnv::getDoclink('article/Differential_User_Guide.html');
+  }
+
   public function getFactObjectsForAnalysis() {
     return array(
       new DifferentialRevision(),

src/docs/userguide/differential_faq.diviner

@@ -13,8 +13,8 @@ This behavior is intentional, to encourage authors to update revisions when they
 make minor changes after a revision is accepted. For example, a reviewer may
 accept a change with a comment like this:
 
-  Looks great, but can you add some documentation for the foo() function
-  before you land it? I also caught a couple typos, see inlines.
+> Looks great, but can you add some documentation for the foo() function
+> before you land it? I also caught a couple typos, see inlines.
 
 If updating the revision reverted the status to "needs review", the author
 is discouraged from updating the revision when they make minor changes because
@@ -23,8 +23,8 @@ they'll have to wait for their reviewer to have a chance to look at it again.
 Instead, the "accept" state is sticky to encourage them to update the revision
 with a comment like:
 
-  ```- Added docs.
-  - Fixed typos.```
+> - Added docs.
+> - Fixed typos.
 
 This makes it much easier for the reviewer to go double-check those changes
 later if they want, and the update tells them that the author acknowledged their
@@ -33,8 +33,8 @@ suggestions even if they don't bother to go double-check them.
 If an author makes significant changes and wants to get them looked at, they can
 always "request review" of an accepted revision, with a comment like:
 
-  When I was testing my typo fix, I realized I actually had a bug, so I had to
-  make some more changes to the bar() implementation -- can you look them over?
+> When I was testing my typo fix, I realized I actually had a bug, so I had to
+> make some more changes to the bar() implementation -- can you look them over?
 
 If authors are being jerks about this (making sweeping changes as soon as they
 get an accept), solve the problem socially by telling them to stop being jerks.
@@ -43,8 +43,8 @@ stopping them from silently changing the code before pushing it, anyway.
 
 = How can I enable syntax highlighting? =
 
-You need to install and configure **Pygments**. Consult the configuration file
-for instructions.
+You need to install and configure **Pygments** to highlight anything else than
+PHP. Consult the configuration file for instructions.
 
 = What do the whitespace options mean? =
 
@@ -60,3 +60,11 @@ Most of these are pretty straightforward, but "Ignore Most" is not:
     can be changed in the `differential.whitespace-matters` configuration
     setting.
   - **Ignore All**: Ignore all whitespace changes in all files.
+
+= What does the very light green and red backgrounds mean? =
+
+Differential uses these colors to mark changes coming from rebase - they are
+part of the diff but they were not added or removed by the author. They can
+appear in diff of diffs against different bases. See
+[[ https://secure.phabricator.com/D3324?vs=6468&id=6513#toc | D3324 ]] for
+example.