For example in the reference page explain that you can't concatenate in the print comand and stuff like that.
The reference pages are going to get very big if they explain all of the things that you can't do.
In any case, if you are determined to (or daft enough) you can concatenate in the print() command.
String Hello = "Hello";
String World = "World";
int number = 123;
void setup()
{
Serial.begin(115200);
Serial.println(Hello + " " + World + " " + (String)number );
}
void loop()
{}
But please don't do it !