Include doc stubs for excluded music methods (#411)

libs/music/docs/reference/music/play-sound-until-done.md

@@ -0,0 +1,3 @@
+# play Sound Until Done
+
+This function is not enabled for @boardname@.

libs/music/docs/reference/music/play-sound.md

@@ -0,0 +1,3 @@
+# play Sound
+
+This function is not enabled for @boardname@.

libs/music/docs/reference/music/sounds.md

@@ -0,0 +1,3 @@
+# sounds
+
+This function is not enabled for @boardname@.