GraphQL description cop misses missing description from declaration methods

The 'missing GraphQL field descriptions' cop lets methods that indirectly declare field via method get away with not having a description.

e.g. markdown_field https://gitlab.com/gitlab-org/gitlab/blob/71b371b9e81a1374e193449a82c9a9a718bfe75c/app/graphql/types/issue_type.rb#L17

Related to: #29974 (closed)