time: warn that RFC3339Nano does not guarantee a natural ordering

Fixes #19635

Change-Id: I85e725dbc85843afd0f4d82f5127fecacc1cb524
Reviewed-on: https://go-review.googlesource.com/47090Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/format.go

@@ -61,6 +61,8 @@ import "errors"
 // 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.
+// The RFC3339Nano format removes trailing zeros from the seconds field
+// and thus may not sort correctly once formatted.
 const (
 	ANSIC       = "Mon Jan _2 15:04:05 2006"
 	UnixDate    = "Mon Jan _2 15:04:05 MST 2006"