In relation to https://jira.mongodb.org/browse/SERVER-20529
it seems that documents in capped collections can no longer shrink in size during an update resulting to errmsg: "Cannot change the size of a document in a capped collection" "code" : 10003
Unfortunately this is not reflected in the manuals yet ( https://docs.mongodb.org/manual/core/capped-collections/ ) where
only mentions that a document can't grow in size.
Since this is breaking backward compatibility it should stated loud and clear in the manual.