docs: document non-pointer receiver for AudioContent (#660)

AudioContent should have had a non-pointer receiver, to be consistent
with other content types. Add this to our log of rough edges to
reconsider for v2.

Fixes #641

Author: findleyr

docs/rough_edges.md

@@ -23,3 +23,6 @@ v2.
     other notification param types.
   - Similarly, `ProgressNotificationParams` should probably have been
     `ProgressParams`.
+
+- `AudioContent.MarshalJSON` should have had a pointer receiver, to be
+  consistent with other content types.

internal/docs/rough_edges.src.md

@@ -22,3 +22,6 @@ v2.
     other notification param types.
   - Similarly, `ProgressNotificationParams` should probably have been
     `ProgressParams`.
+
+- `AudioContent.MarshalJSON` should have had a pointer receiver, to be
+  consistent with other content types.