audio: Clarify "mute" behavior for playback parameters

When the client specifies `TimestretchFallbackMode.MUTE`,
it should still be allowed for the HAL implementation
to return an error if parameters are out of range and
it does not support muted playback.

Bug: 381385909
Bug: 384626226
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I757856dadd9cf234daa3554b0ca33e7b4e2e4f00

media/aidl/android/media/audio/common/AudioPlaybackRate.aidl

diff --git a/media/aidl/android/media/audio/common/AudioPlaybackRate.aidl b/media/aidl/android/media/audio/common/AudioPlaybackRate.aidl
index 3dd474f..45921d5 100644
--- a/media/aidl/android/media/audio/common/AudioPlaybackRate.aidl
+++ b/media/aidl/android/media/audio/common/AudioPlaybackRate.aidl
@@ -71,9 +71,15 @@
         SYS_RESERVED_CUT_REPEAT = -1,
         /** Reserved for use by the framework. */
         SYS_RESERVED_DEFAULT = 0,
-        /** Play silence for parameter values that are out of range. */
+        /**
+         * If possible, play silence for parameter values that are out of range,
+         * otherwise return an error (same as 'FAIL' would return).
+         */
         MUTE = 1,
-        /** Return an error while trying to set the parameters. */
+        /**
+         * Always return an error while trying to set the parameters that are
+         * out of range.
+         */
         FAIL = 2,
     }
     /**