time: document that RFC822/1123 don't parse all RFC formats

Fixes #14505.

Change-Id: I46196b26c9339609e6e3ef9159de38c5b50c2a1b
Reviewed-on: https://go-review.googlesource.com/23922
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/time/format.go

@@ -51,6 +51,9 @@ import "errors"
 // use of "GMT" in that case.
 // In general RFC1123Z should be used instead of RFC1123 for servers
 // that insist on that format, and RFC3339 should be preferred for new protocols.
+// RFC822, RFC822Z, RFC1123, and RFC1123Z are useful for formatting;
+// when used with time.Parse they do not accept all the time formats
+// permitted by the RFCs.
 const (
 	ANSIC       = "Mon Jan _2 15:04:05 2006"
 	UnixDate    = "Mon Jan _2 15:04:05 MST 2006"