Scope
Restore documentation describing preconditions of bson_t out-params added in CDRIVER-3368.

Background
CDRIVER-3368 added documentation describing preconditions of bson_t out-params. It is still visible on older branches: https://mongoc.org/libbson/1.26.0/lifetimes.html

This was removed from mongoc.org docs as part of CDRIVER-5565 to migrate to mongodb.com docs.

The mongodb.com docs were later removed as part of DOCSP-38883. As a result, links on mongoc.org for "Overwritable Storage" redirect to https://www.mongodb.com/docs/404/#overwritable-storage.
Since this is an API precondition, I think this is a better fit for the mongoc.org API docs.