Minor - P4 The upsertSupplied option to update statements says stability: stable but it an internal implementation detail of $merge.
upsertSupplied:
description: "Only applicable when upsert is true. If set, and if no documents match
the query, the update subsystem will insert the document supplied as
'c.new' rather than generating a new document from the update spec."
type: optionalBool
stability: stable