Fix doc nits

Many tiny changes to stdlib doc comments to make them consistent (for example
"Returns foo", rather than "Return foo", per RFC1574), adding missing periods, paragraph
breaks, backticks for monospace style, and other minor nits.

https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text

library/core/src/array/ascii.rs

@@ -2,7 +2,7 @@ use crate::ascii;
 
 #[cfg(not(test))]
 impl<const N: usize> [u8; N] {
-    /// Converts this array of bytes into a array of ASCII characters,
+    /// Converts this array of bytes into an array of ASCII characters,
     /// or returns `None` if any of the characters is non-ASCII.
     ///
     /// # Examples
@@ -29,7 +29,7 @@ impl<const N: usize> [u8; N] {
         }
     }
 
-    /// Converts this array of bytes into a array of ASCII characters,
+    /// Converts this array of bytes into an array of ASCII characters,
     /// without checking whether they're valid.
     ///
     /// # Safety

library/core/src/array/iter.rs

@@ -47,8 +47,10 @@ impl<T, const N: usize> IntoIterator for [T; N] {
     type IntoIter = IntoIter<T, N>;
 
     /// Creates a consuming iterator, that is, one that moves each value out of
-    /// the array (from start to end). The array cannot be used after calling
-    /// this unless `T` implements `Copy`, so the whole array is copied.
+    /// the array (from start to end).
+    ///
+    /// The array cannot be used after calling this unless `T` implements
+    /// `Copy`, so the whole array is copied.
     ///
     /// Arrays have special behavior when calling `.into_iter()` prior to the
     /// 2021 edition -- see the [array] Editions section for more information.