audiohal: Fix typos in comments, add missing comments

These changes are in comments only, no ABI breaking changes.

Bug: 32238101
Test: make
Change-Id: Iead4ba01dae1a1c05d8fbed7f194182128e9e6ad
diff --git a/audio/effect/2.0/IDownmixEffect.hal b/audio/effect/2.0/IDownmixEffect.hal
index 06409a3..1232095 100644
--- a/audio/effect/2.0/IDownmixEffect.hal
+++ b/audio/effect/2.0/IDownmixEffect.hal
@@ -25,7 +25,13 @@
         FOLD   // mix the extra channels with FL/FR
     };
 
+    /**
+     * Sets the current downmix preset.
+     */
     setType(Type preset) generates (Result retval);
 
+    /**
+     * Gets the current downmix preset.
+     */
     getType() generates (Result retval, Type preset);
 };