doc: clarify how to verify changes locally

It was unclear that running the App Engine version
would use play.golang.org to run the code examples.

This clarifies this, as well as elaborating on the
distinction between the App Engine version and the
local version.

Change-Id: I3f6fab3669c4d37b0b4d5a3d227f2e61ce162092
Reviewed-on: https://go-review.googlesource.com/15907
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed