Add documentation for the global blank literal

[westonganger]

I was going to make a PR for blank? and present? filters but then found this super handy blank literal.

{% if section.settings.description != blank %}
  <p>{{ section.settings.description }}</p>
{% endif %}

This should be added to the documentation somewhere.

Shopify/liquid#223 (comment)

https://github.com/Shopify/liquid/blob/master/lib/liquid/expression.rb#L20