html/template, text/template: clarify template redefinition behavior

html/template, text/template: clarify template redefinition behavior

Make two important points clearer:

 - Giving a template definition containing
   nothing but spaces has no effect.
 - Giving a template definition containing
   non-spaces can only be done once per template.

Fixes #16912.
Fixes #16913.
Fixes #17360.

Change-Id: Ie3971b83ab148b7c8bb800fe4a21579566378e3e
Reviewed-on: https://go-review.googlesource.com/31459
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>