cleanup: add/fix comments

error_thunk.go

@@ -1,9 +1,11 @@
 package xmlsec
 
 // onError_cgo is a C function that can be passed to xmlSecErrorsSetCallback which
-// in turn invokes the go function onError which captures the error.
+// in turn invokes the go function onError which captures errors generated by
+// libxmlsec.
+//
 // For reasons I do not completely understand, it must be defined in a different
-// file from onError
+// file from onError.
 
 // void onError_cgo(char *file, int line, char *funcName, char *errorObject, char *errorSubject, int reason, char *msg) {
 //   onError(file, line, funcName, errorObject, errorSubject, reason, msg);