Added examples for use of Mode, Whitespace, and IsIdentRune properties.

Fixes #23768

Change-Id: I2528e14fde63a4476f3c25510bf0c5b73f38ba5d
Reviewed-on: https://go-review.googlesource.com/93199
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>