Add explanatory comment for new code

Author: ChaiTRex

library/core/src/num/uint_macros.rs

@@ -1491,6 +1491,8 @@ macro_rules! uint_impl {
                       without modifying the original"]
         #[inline]
         pub const fn checked_ilog(self, base: Self) -> Option<u32> {
+            // Inform compiler of optimizations when the base is known at
+            // compile time and there's a cheaper method available.
             if core::intrinsics::is_val_statically_known(base) {
                 if base == 2 {
                     return self.checked_ilog2();