Adds docs for QueryRecorder tests

What does this MR do?

Adds documentation describing use of QueryRecorder to avoid regressions of N+1 queries problems

Closes #28492 (closed)

Preview

• https://gitlab.com/gitlab-org/gitlab-ce/blob/28492-add-dev-documentation-for-using-the-queryrecorder-in-specs/doc/development/query_recorder.md