Clarify the reason u8 is used instead of u7

Author: wooster0

lib/std/ascii.zig

@@ -1,13 +1,11 @@
-//! The ASCII character encoding standard.
+//! The 7-bit [ASCII](https://en.wikipedia.org/wiki/ASCII) character encoding standard.
+//!
+//! This is not to be confused with the 8-bit [Extended ASCII](https://en.wikipedia.org/wiki/Extended_ASCII).
+//!
+//! The reason `u8` is used as the type instead of `u7` is convenience.
 //!
 //! See also: https://en.wikipedia.org/wiki/ASCII#Character_set
 
-// I could have taken only a u7 to make this clear, but it would be slower
-// It is my opinion that encodings other than UTF-8 should not be supported.
-//
-// (and 128 bytes is not much to pay).
-// Also does not handle Unicode character classes.
-
 const std = @import("std");
 const mem = std.mem;
 const testing = std.testing;