docs: clarify join behavior on objects

[ded-furby]

Closes #3541

Summary

• clarify that join/1 accepts object input as well as arrays
• explain that object input is processed via the values produced by .[]
• tighten the wording so the unsupported case is arrays/objects among the joined values

Testing

• verified the current behavior locally with jq -nr '{"b":1,"a":2} | join(",")' and jq -nr '[[1,2],[3,4]] | try join(",") catch .'\n- ran git diff --check