time: improve comments about valid layouts being invalid Parse values

Updates #9346
Updates #22135

Change-Id: I7039c9f7d49600e877e35b7255c341fea35890e2
Reviewed-on: https://go-review.googlesource.com/74890Reviewed-by: Rob Pike <r@golang.org>

src/time/example_test.go

@@ -317,8 +317,8 @@ func ExampleParse() {
 	t, _ = time.Parse(shortForm, "2013-Feb-03")
 	fmt.Println(t)
 
-	// Valid layouts may not be a valid time value, due to format specifiers
-	// like _ for zero padding or Z for zone information.
+	// Some valid layouts are invalid time values, due to format specifiers
+	// such as _ for zero padding and Z for zone information.
 	// For example the RFC3339 layout 2006-01-02T15:04:05Z07:00
 	// contains both Z and a time zone offset in order to handle both valid options:
 	// 2006-01-02T15:04:05Z

src/time/format.go

@@ -18,8 +18,8 @@ import "errors"
 // reference time looks like so that the Format and Parse methods can apply
 // the same transformation to a general time value.
 //
-// Valid layouts may not be a valid time value for time.Parse, due to formats
-// like _ for zero padding or Z for zone information.
+// Some valid layouts are invalid time values for time.Parse, due to formats
+// such as _ for zero padding and Z for zone information.
 //
 // Within the format string, an underscore _ represents a space that may be
 // replaced by a digit if the following number (a day) has two digits; for