Expand docs a little bit

Author: phansch

clippy_lints/src/missing_const_for_fn.rs

@@ -11,11 +11,13 @@ use syntax_pos::Span;
 
 /// **What it does:**
 ///
-/// Suggests the use of `const` in functions and methods where possible
+/// Suggests the use of `const` in functions and methods where possible.
 ///
 /// **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.
+/// 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.
 ///
 /// **Known problems:**
 ///