Fix documentation incorrectness

Author: phansch

clippy_lints/src/missing_const_for_fn.rs

@@ -15,9 +15,7 @@ use syntax_pos::Span;
 ///
 /// **Why is this bad?**
 ///
-/// Not using `const` is a missed optimization. Instead of having the function execute at runtime,
-/// when using `const`, it's evaluated at compiletime. The result of the evaluation is inlined
-/// during compilation and doesn't have to be recomputed during runtime.
+/// Not having the function const prevents callers of the function from being const as well.
 ///
 /// **Known problems:**
 ///