unicode/utf8: add link to formal UTF-8 description.

Fixes #31590

Change-Id: I7fd6dcc5c34496776439ff0295f18b5fb5cb538a
Reviewed-on: https://go-review.googlesource.com/c/go/+/199141Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/unicode/utf8/utf8.go

@@ -4,6 +4,7 @@
 
 // Package utf8 implements functions and constants to support text encoded in
 // UTF-8. It includes functions to translate between runes and UTF-8 byte sequences.
+// See https://en.wikipedia.org/wiki/UTF-8
 package utf8
 
 // The conditions RuneError==unicode.ReplacementChar and