time: note that formats may parse invalid strings

The existing documentation for time format constants doesn't mention
that they may parse technically-invalid strings, such as single-digit
hours when a two-digit hour is required by a specification.  This commit
adds a short warning note to that effect.

Fixes #37616

Change-Id: I6e5e12bd42dc368f8ca542b4c0527a2b7d30acaf
Reviewed-on: https://go-review.googlesource.com/c/go/+/229460
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/format.go

@@ -67,7 +67,7 @@ import "errors"
 // that insist on that format, and RFC3339 should be preferred for new protocols.
 // RFC3339, 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.
+// permitted by the RFCs and they do accept time formats not formally defined.
 // The RFC3339Nano format removes trailing zeros from the seconds field
 // and thus may not sort correctly once formatted.
 const (