Update PowerHAL documentation to clarify Session ID size restriction

Test: n/a
Bug: 315894228
Change-Id: I69ee157bc79e26578bd13a907662ac8756d7ce47

power/aidl/android/hardware/power/SessionConfig.aidl

@@ -25,6 +25,9 @@ parcelable SessionConfig {
     /**
      * The session's unique ID, used to identify the session for debugging and
      * for multiplexing on the per-process FMQ channel.
+     *
+     * Values that fit in the 32-bit int range value must be provided when using
+     * the FMQ API, as the FMQ messages can only accept 32-bit Session IDs.
      */
     long id;
 }