Clarify compressed sounds;

api/lovr/data/Sound/init.lua

@@ -24,7 +24,7 @@ return {
     MP3 are compressed audio formats.  When creating a sound from a compressed format, there is an
     option to immediately decode it, storing it uncompressed in memory.  It can be a good idea to
     decode short sound effects, since they won't use very much memory even when uncompressed and it
-    will improve CPU usage.
+    will improve CPU usage.  Compressed sounds can not be written to using `Sound:setFrames`.
 
     Streams
     ---

api/lovr/data/Sound/isCompressed.lua

@@ -3,7 +3,7 @@ return {
   description = [[
     Returns whether the Sound is compressed.  Compressed sounds are loaded from compressed audio
     formats like MP3 and OGG.  They use a lot less memory but require some extra CPU work during
-    playback.
+    playback.  Compressed sounds can not be modified using `Sound:setFrames`.
   ]],
   arguments = {},
   returns = {