Document Structures in GraphQL Types.

[dopry]

The structures PositiveInt, QuerySetList, TagList, PaginationType, BasePaginatedType and type factory PaginatedQuerySet should be documented in docs/general-usage/graphql-types.rst and how they are used by the decorators should be documented in docs/general-usage/decorators.rst.

This would be a good starter issue as it will help new devs become familiar with the wagtail-grapple architecture and code base.