fix clip op doc operation (#6314)

Author: kexinzhao

paddle/operators/clip_op.cc

@@ -52,7 +52,11 @@ class ClipOpMaker : public framework::OpProtoAndCheckerMaker {
 Clip Operator.
 
 The clip operator limits the value of given input within an interval. The interval is
-specified with arguments 'min' and 'max'.
+specified with arguments 'min' and 'max':
+
+$$
+Out = \min(\max(X, min), max)
+$$
 
 )DOC");
   }