Document how to enable eager loading in specs via `GITLAB_TEST_EAGER_LOAD`

The following discussion from !72964 (merged) should be addressed:

• @splattael started a discussion: (+2 comments)

  Suggestion (non-blocking) We've never documented the variable GITLAB_TEST_EAGER_LOAD so I was wondering if we should add a small section to https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#rspec just in case folks want rely on eager loading? 🤔