docs(x/auth): improve godoc for GetSigningTxData (#16933)

x/auth/tx/adapter.go

@@ -13,7 +13,10 @@ import (
 )
 
 // GetSigningTxData returns an x/tx/signing.TxData representation of a transaction for use in the signing
-// API defined in x/tx.
+// API defined in x/tx.  The reason for all of this conversion is that x/tx depends on the protoreflect API
+// defined in google.golang.org/protobuf while x/auth/tx depends on the legacy proto API defined in
+// github.com/gogo/protobuf and the downstream SDK fork of that library, github.com/cosmos/gogoproto.
+// Therefore we need to convert between the two APIs.
 func (w *wrapper) GetSigningTxData() txsigning.TxData {
 	body := w.tx.Body
 	authInfo := w.tx.AuthInfo