net/mail: clarify doc comment

Rewrite the text added in CL 50911, which I did not understand.

Change-Id: Id6271ffe2f7c8833dd7733fe0254fa4927fac150
Reviewed-on: https://go-review.googlesource.com/78124
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/mail/message.go

@@ -13,7 +13,7 @@ Notable divergences:
 	* The full range of spacing (the CFWS syntax element) is not supported,
 	  such as breaking addresses across lines.
 	* No unicode normalization is performed.
-	* Address with some RFC 5322 3.2.3 specials without quotes are parsed.
+	* The special characters ()[]:;@\, are allowed to appear unquoted in names.
 */
 package mail