approval-distribution: limit the amount of assignments on unify (#2737)

* approval-distribution: limit the amount of packets on unify

* guide: fix a typo

* compilation fix

* grammar

* Update roadmap/implementers-guide/src/node/approval/approval-distribution.md

Co-authored-by: David <[email protected]>

* more grammar

* propagate only local assignments/approvals after a certain depth

* increase the threshold

* guides update

Co-authored-by: David <[email protected]>