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

core/java/android/companion/AssociationRequest.java

@@ -127,6 +127,12 @@ public final class AssociationRequest implements Parcelable {
         public Builder() {}
 
         /**
+         * Whether only a single device should match the provided filter.
+         *
+         * When scanning for a single device with a specifc {@link BluetoothDeviceFilter} mac
+         * address, bonded devices are also searched among. This allows to obtain the necessary app
+         * privileges even if the device is already paired.
+         *
          * @param singleDevice if true, scanning for a device will stop as soon as at least one
          *                     fitting device is found
          */