Previously our GraphQL styleguide stated that description strings should
not end in periods. Descriptions provided by `ruby-graphql` objects all
do end in periods, so our overall API is inconsistent in how it presents
its schema descriptions.

This change switches our description styleguide to recommend that we add
periods to the end of description strings.

It also adds to the existing `GraphQL/Descriptions` cop to enforce this
which can auto-correct offenses.