Clarifications to extending filtering on relationships

[ml-evs]

#524 was merged in anticipation of v1.3.0-rc.1, but we want to clarify some parts of it before the final 1.3.0 release (which should also be soon!)

We can check through @ndaelman-hu's comments for a good starting point:

• Allow extended filtering on relationships #524 (comment)
• Allow extended filtering on relationships #524 (comment)
• Allow extended filtering on relationships #524 (comment)
• Allow extended filtering on relationships #524 (comment)
• Allow extended filtering on relationships #524 (comment)