Clarify that functions/procedures are dropped when any extension that
depends on them is dropped.

Reported-by: David G. Johnston

Discussion: https://postgr.es/m/CAKFQuwbPSHMDGkisRUmewopweC1bFvytVqB=a=X4GFg=4ZWxPA@mail.gmail.com

Backpatch-through: 13