Document alternate remarkup headers

Summary: Companion to D5854. Ref T2849.

Test Plan: {F42641}

Reviewers: btrahan, jevripio

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2849

Differential Revision: https://secure.phabricator.com/D5855

Author: epriestley

src/docs/userguide/remarkup.diviner

@@ -33,8 +33,17 @@ These are block styles, and must be separated from surrounding text by
 empty lines:
 
   = Large Header =
+
   == Smaller Header ==
+
+  Also a Large Header
+  ===================
+
+  Also a Smaller Header
+  ---------------------
+
   > Quoted Text
+
   Use "- " or "* " for bulleted lists, and "# " for numbered lists.
   Use ``` or indent two spaces for code.
   Use %%% for a literal block.
@@ -63,7 +72,13 @@ Make **headers** like this:
 
   ===== Very Small Header =====
 
-You can optionally omit the trailing "=" signs -- that is, these are the same:
+  Alternate Large Header
+  ======================
+
+  Alternate Smaller Header
+  ------------------------
+
+You can optionally omit the trailing `=` signs -- that is, these are the same:
 
   == Smaller Header ==