Clarify server handling of related topics

[GeorgDangl]

This came up from @pasi-paasiala

We should update the documentation / README to ensure that it's clear that servers that support topic relations are always expected to return all the relations when getting topic data, so that a client would never have to enumerate all other topics to check if a relation exists.
Meaning, if we query the server to get a "child" topic, it will include the "parent" topic in it's 'related_topics' list (and vice versa).