Libbson is pretty aggressive about warning the consumer of invalid API
use. However, that means that we short circuit some functions. This is
valid since consumers shipping invalid API use is nothing we can protect
against, but this will at least quiet some static analyzers.

Add blurb on fast mutation.

This function is helpful to copy a bson document but not all of the
elements. Equivalent to a removal of a key from a document (which we
do not currently allow in bson_t since it is append/overwrite only).

This function will copy the current field that iter is pointing at
to the next field in the bson_t. You can override the name if you
so choose.

These just happened to be missing.