time: document that only Jan and January, Mon and Monday are recognized

Not "J", not "JAN", not "jan", etc.

Fixes #17523.

Change-Id: I16b5da97e73d88c6680c36401d30f8a195061527
Reviewed-on: https://go-review.googlesource.com/32636
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/time/format.go

@@ -42,6 +42,13 @@ import "errors"
 //	Z07:00 Z or ±hh:mm
 //	Z07    Z or ±hh
 //
+// The recognized day of week formats are "Mon" and "Monday".
+// The recognized month formats are "Jan" and "January".
+//
+// Text in the format string that is not recognized as part of the reference
+// time is echoed verbatim during Format and expected to appear verbatim
+// in the input to Parse.
+//
 // The executable example for time.Format demonstrates the working
 // of the layout string in detail and is a good reference.
 //