time: update docs to clarify goroutine-safety expectations of Time

Fixes #19935

Change-Id: I28e4cd6924f1b965a429a6a5ebfc4c0f952e8ae6
Reviewed-on: https://go-review.googlesource.com/40390Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/time.go

@@ -81,8 +81,11 @@ import "errors"
 //
 // Programs using times should typically store and pass them as values,
 // not pointers. That is, time variables and struct fields should be of
-// type time.Time, not *time.Time. A Time value can be used by
-// multiple goroutines simultaneously.
+// type time.Time, not *time.Time.
+//
+// A Time value can be used by multiple goroutines simultaneously except
+// that the methods GobDecode, UnmarshalBinary, UnmarshalJSON and
+// UnmarshalText are not concurrency-safe.
 //
 // Time instants can be compared using the Before, After, and Equal methods.
 // The Sub method subtracts two instants, producing a Duration.