Rollup merge of #139701 - Rudxain:doc-pow2, r=tgross35

docs: clarify uint exponent for `is_power_of_two`

This makes the documentation more explicit for that method. I know this might seem "nit-picky", but `k` could be interpreted as "any Real or Complex number". A trivial example would be $`3 = 2^{log_2(3)}`$ which "proves that three is a power of two" (according to that vague definition).

BTW, when I read the implementation, I was surprised to see that `1` is considered a power of 2 despite being odd (it does make sense in some contexts, but still not intuitive). So I wrote "positive int" before correcting it to "unsigned int"

library/core/src/num/uint_macros.rs

@@ -3343,7 +3343,7 @@ macro_rules! uint_impl {
             }
         }
 
-        /// Returns `true` if and only if `self == 2^k` for some `k`.
+        /// Returns `true` if and only if `self == 2^k` for some unsigned integer `k`.
         ///
         /// # Examples
         ///