Add rule doc notice type for rule description

[bmish]

This is helpful so that one can use the rule name as the rule doc title and then ensure the rule description is included below the rule doc title as one of the notices.

# rule-name

Rule description. This is the new notice type.

<!-- end auto-generated rule header -->

...

It's possible at some point we might want to show this description in italics to differentiate it from the rest of the rule doc body, or allow the user to specify their own formatting function for it.

Helps with #347. In the future, we might make this notice a default notice type if we change the default rule doc title format to exclude the rule description.

Closes #192 because this provides a way to always include the rule description as a notice in the rule doc header.