Auto merge of #26878 - Esption:master, r=pnkfelix

I noticed in docs, specifically http://doc.rust-lang.org/std/primitive.u8.html#method.is_power_of_two, that it was like this, and it was apparently in multiple places too.

Didn't change any occurrences through the cross-depo things. There's a lot in /src/llvm/ for instance, but I'm not confident on how to go about sending fixes for those, so this is just what's in the base rust depo.

r? @steveklabnik

src/libcore/num/mod.rs

@@ -1126,7 +1126,7 @@ macro_rules! uint_impl {
             acc
         }
 
-        /// Returns `true` iff `self == 2^k` for some `k`.
+        /// Returns `true` if and only if `self == 2^k` for some `k`.
         #[stable(feature = "rust1", since = "1.0.0")]
         #[inline]
         pub fn is_power_of_two(self) -> bool {