Add documentation for cowsay

Summary:
Cowsay is great, but what if you want to talk like a koala?

This diff adds diviner documentation on how to use the additional templates in externals/cowsay/cows/

Test Plan:
  bin/diviner generate
	https://my.phorge.site/book/phorge/article/differential_cowsay/

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25434

src/docs/user/userguide/differential.diviner

@@ -66,6 +66,7 @@ Continue by:
   - diving into the details of inline comments in
     @{article:Differential User Guide: Inline Comments}; or
   - reading the FAQ at @{article:Differential User Guide: FAQ}; or
+  - learning how to use markup in comments at @{article:Remarkup Reference}; or
   - learning about test plans in
     @{article:Differential User Guide: Test Plans}; or
   - learning more about Herald in @{article:Herald User Guide}.

src/docs/user/userguide/differential_faq.diviner

@@ -41,8 +41,9 @@ always "request review" of an accepted revision, with a comment like:
 
 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.
-Unless you've configured additional layers of enforcement, there's nothing
-stopping them from silently changing the code before pushing it, anyway.
+Unless you've configured additional layers of enforcement (by
+using @{article:Herald}), there's nothing stopping them from silently changing
+the code before pushing it, anyway.
 
 
 = How can I enable syntax highlighting? =
@@ -50,6 +51,10 @@ stopping them from silently changing the code before pushing it, anyway.
 You need to install and configure **Pygments** to highlight anything else than
 PHP. See the `pygments.enabled` configuration setting.
 
+= What formatting can be used in comments? =
+
+Phorge implements a markup language similar to other markup languages like
+Markdown and Wiki markup. See @{article:Remarkup Reference}.
 
 = What do the very light green and red backgrounds mean? =
 

src/docs/user/userguide/remarkup.diviner

@@ -732,3 +732,7 @@ to the first anchor with "xyz" as a prefix of the anchor name.
 Remarkup editors provide a fullscreen composition mode. This can make it easier
 to edit large blocks of text, or improve focus by removing distractions. You can
 exit **Fullscreen** mode by clicking the button again or by pressing escape.
+
+See Also
+========
+* @{article:Remarkup Reference: Cowsay}

src/docs/user/userguide/remarkup_cowsay.diviner

@@ -0,0 +1,61 @@
+@title Remarkup Reference: Cowsay
+@group userguide
+
+Overview
+--------
+
+Cowsay is an application by Tony Monroe which has been ported over to
+Phabricator/Phorge to allow your comments to be voiced by
+a cow.
+
+Basic Usage
+-----------
+
+A basic example of using cowsay would be to add a comment
+  cowsay{{{Great work!}}}
+which generates:
+```
+ _____________
+< Great work! >
+ -------------
+        \   ^__^
+         \  (oo)\_______
+            (__)\       )\/\
+                ||----w |
+                ||     ||
+```
+
+Other Cowsay Templates
+----------------------
+Other templates are available in externals/cowsay/cows/, and you can specify
+one by calling cowsay with the 'cow' parameter. eg:
+  cowsay(cow='tux'){{{Great work!}}}
+generates
+```
+ _____________
+< Great work! >
+ -------------
+   \
+    \
+        .--.
+       |o_o |
+       |:_/ |
+      //   \ \
+     (|     | )
+    /'\_   _/`\
+    \___)=(___/
+```
+
+Other Parameters
+----------------
+* think (set to 1 for thinking bubbles)
+* eyes (default 'oo')
+* tongue (default '  ', try 'P')
+
+See Also
+========
+* @{article:Remarkup Reference}
+* Have you tried figlet:
+```
+figlet{{{figlet generates big text!}}}
+```