net/textproto: document enforcement of RFC 9112 for headers

Fixes #68590

Change-Id: Ie7cf1fe8379182f86317d5ebb7f45a404ecd70e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/601555
Auto-Submit: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: seankhliao

src/net/textproto/reader.go

@@ -642,8 +642,8 @@ func (r *Reader) upcomingHeaderKeys() (n int) {
 // the rest are converted to lowercase. For example, the
 // canonical key for "accept-encoding" is "Accept-Encoding".
 // MIME header keys are assumed to be ASCII only.
-// If s contains a space or invalid header field bytes, it is
-// returned without modifications.
+// If s contains a space or invalid header field bytes as
+// defined by RFC 9112, it is returned without modifications.
 func CanonicalMIMEHeaderKey(s string) string {
 	// Quick check for canonical encoding.
 	upper := true

src/net/textproto/textproto.go

@@ -5,6 +5,9 @@
 // Package textproto implements generic support for text-based request/response
 // protocols in the style of HTTP, NNTP, and SMTP.
 //
+// This package enforces the HTTP/1.1 character set defined by
+// RFC 9112 for header keys and values.
+//
 // The package provides:
 //
 // [Error], which represents a numeric error response from