Move schema description validation rules into general validator

Created by: NotSpecial

We use additional schema rules such as 'description' to make our schema more descriptive. However, Cerberus will complain about unknown schema rules, so they have to be defined somewhere, even if they don't do anything.

Currently, this is done in the documentation sub-module. This is a bit problematic, as our API is designed with replaceable modules in mind -- but deactivating or replacing the doc module would now cause errors in all schemas.

As the additional description rules are important whether or not we use the online docs, I have moved the rules into the general validator.