Document condition when CDM looks at bonded devices

As reported in attached bug, there's been some confusion around which
specific kind of request does or does not include bonded devices when
searching.

This mentions the exact condition, and explains the use case when user
might want to take advantage of it.

Fixes: 139376317
Test: presubmit
Change-Id: I98501a235a044a0e184dcbb2363c158cc0d0882c