Rewrite AsciiStr rustdoc

Author: tormol

src/ascii_str.rs

@@ -6,7 +6,9 @@ use AsciiCast;
 use ascii::Ascii;
 use ascii_string::AsciiString;
 
-/// A borrowed ascii string, like a slice into an `AsciiString`.
+/// A str with only ASCII codepoints.
+///
+/// Can either be borrowed from an `AsciiString`, or a validated `str` / `[u8]`.
 #[derive(PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct AsciiStr {
     slice: [Ascii],