added note to re constructor regarding performance (#13224)
added note to re constructor regarding performance (#13224)

Since I was new to regex I did not know that there is a compilation going on with ``re"[abc]"`` constructor and so I followed the other examples in the docs blindly, that is I just put the constructor directly in the arguments of match, find, etc., which was inside a loop and then wondered why my performance was so bad. Of course putting it outside the loop made it vastly more performant. People like me would benefit from the small note I added I would think :)
dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim