time: document the Time.String is meant for debugging

Fixes #20876

Change-Id: Ic62c4f59e3ddcae891aa9526f9693d233dd524fc
Reviewed-on: https://go-review.googlesource.com/47552
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/time/format.go

@@ -430,6 +430,10 @@ func formatNano(b []byte, nanosec uint, n int, trim bool) []byte {
 // If the time has a monotonic clock reading, the returned string
 // includes a final field "m=±<value>", where value is the monotonic
 // clock reading formatted as a decimal number of seconds.
+//
+// The returned string is meant for debugging; for a stable serialized
+// representation, use t.MarshalText, t.MarshalBinary, or t.Format
+// with an explicit format string.
 func (t Time) String() string {
 	s := t.Format("2006-01-02 15:04:05.999999999 -0700 MST")