Clearer docs when it is OK for maintainer to squash

What does this MR do?

See discussion thread in slack https://gitlab.slack.com/archives/C02PF508L/p1543315691879400

I'm hoping to highlight the author's (MR creator) preference. Ultimately the final judgement call should be made by the maintainer but I think that if the author is putting effort into keeping their commit history clean then you should probably have a good reason to overrule their preference and squash their work down.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.