Refine broadcastradio HAL comments.

Bug: b/64113304
Test: it builds
Change-Id: I42c1d32dbe436c97f0a05b4fdb88f60f11ceb0a1
diff --git a/broadcastradio/1.1/ITuner.hal b/broadcastradio/1.1/ITuner.hal
index a5c569a..cc2e58d 100644
--- a/broadcastradio/1.1/ITuner.hal
+++ b/broadcastradio/1.1/ITuner.hal
@@ -84,6 +84,13 @@
      * subsequent calls to startBackgroundScan, issuing a single
      * backgroundScanComplete callback.
      *
+     * If a device supports continuous background scanning, it may succeed
+     * (return OK and call backgroundScanComplete) without any additional
+     * operation performed.
+     *
+     * Foreground scanning may be implemented in the front end app with
+     * @1.0::ITuner scan operation.
+     *
      * @return result OK if the scan was properly scheduled (this does not mean
      *                it successfully finished).
      *                UNAVAILABLE if the background scan is unavailable,