Add popcount and popcnt as doc aliases for count_ones methods.

Integer types have a `count_ones` method that end up calling
`intrinsics::ctpop`.
On some architectures, that intrinsic is translated as a corresponding
CPU instruction know as "popcount" or "popcnt".

This PR makes it so that searching for those names in rustdoc shows those methods.

CC https://blog.rust-lang.org/2020/11/19/Rust-1.48.html#adding-search-aliases

library/core/src/num/int_macros.rs

@@ -92,6 +92,8 @@ $EndFeature, "
 "),
             #[stable(feature = "rust1", since = "1.0.0")]
             #[rustc_const_stable(feature = "const_int_methods", since = "1.32.0")]
+            #[doc(alias = "popcount")]
+            #[doc(alias = "popcnt")]
             #[inline]
             pub const fn count_ones(self) -> u32 { (self as $UnsignedT).count_ones() }
         }