Update wording on pure function definition.

I was originally going to put a PR to fix "has not side-effects", but I also re-read this sentence and realized it could be interpreted as saying a pure function returns its parameters, not a deterministic value based on its parameters.

website/docs/concepts/function-components/pure-components.mdx

@@ -4,7 +4,7 @@ title: "Pure Components"
 
 ## Pure function definition
 
-A function is considered pure when the return values are always identical to passed down parameters. And it has not side-effects.
+A function is considered pure when the return values are deterministic given the same parameters, and it has no side-effects.
 
 ## Pure components