Add documentation for custom tag blocks (#7359)

Merge pull request 7359

docs/_docs/plugins/tags.md

@@ -70,3 +70,46 @@ And we would get something like this on the page:
 ```html
 <p>page rendered at: Tue June 22 23:38:47 –0500 2010</p>
 ```
+
+## Tag Blocks
+
+The `render_time` tag seen above can also be rewritten as a tag block by 
+inheriting the `Liquid::Block` class. Look at the example below:
+
+```ruby
+module Jekyll
+  class RenderTimeTagBlock < Liquid::Block
+
+    def render(context)
+      text = super
+      "<p>#{text} #{Time.now}</p>"
+    end
+
+  end
+end
+
+Liquid::Template.register_tag('render_time', Jekyll::RenderTimeTagBlock)
+```
+
+We can now use the tag block anywhere:
+
+{% raw %}
+```liquid
+{% render_time %}
+page rendered at:
+{% endrender_time %}
+```
+{% endraw %}
+
+And we would still get the same output as above on the page:
+
+```html
+<p>page rendered at: Tue June 22 23:38:47 –0500 2010</p>
+```
+
+<div class="note info">
+  <p>In the above example, the tag block and the tag are both registered with 
+  the name <code>render_time</code> but to register a tag and a tag block using 
+  the same name in the same project is not recommended as this may lead to 
+  conflicts.</p>
+</div>