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