Document criteria for writing integration or capybara feature tests

The following discussion from !17545 (merged) should be addressed to add some reasoning behind when feature tests should be added:

When you should strongly consider implementing a full feature integration spec:

* When a feature is across multiple components.
* When a feature is very complex to setup before being able to review, debug, or develop in that area of code.

improve https://docs.gitlab.com/ee/development/testing_guide/testing_levels.html#white-box-tests-at-the-system-level-formerly-known-as-system--feature-tests