Diviner: Improve documentation for remarkup code blocks

Summary:
Improve remarkup reference documentation.

* Added a heading for `Code Blocks`
* Added a sentence mentioning the ##```lang## syntax which was added in {T15481}

Test Plan:
* Run `bin/diviner generate` to regen diviner docs.
* Check out the code blocks section of [[/book/phorge/article/remarkup/#code-blocks|Remarkup Reference]]

Reviewers: valerio.bozzolan, O1 Blessed Committers

Reviewed By: valerio.bozzolan, O1 Blessed Committers

Subscribers: tobiaswiese, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25547

src/docs/user/userguide/remarkup.diviner

@@ -196,6 +196,9 @@ When rendered, this produces:
   - [X] Preheat oven to 450 degrees.
   - [ ] Zest 35 lemons.
 
+Code Blocks
+===========
+
 Make **code blocks** by indenting two spaces:
 
   f(x, y);
@@ -211,6 +214,9 @@ You can specify a language for syntax highlighting with `lang=xxx`:
   lang=html
   <a href="#">...</a>
 
+When using fenced code blocks (triple backticks) you can simply append the
+language right after the backticks, like this: ##```html##
+
 This will highlight the block using a highlighter for that language, if one is
 available (in most cases, this means you need to configure Pygments):