cease docimpact emails, create bugs only

We've been running the DocImpact bug generation code for
several weeks now and are happy with the results. We'd now
like to cease the emails and only generate bugs from DocImpact
strings.

This patch updates the logic to never send an email for DocImpact,
and changes the docstring to reflect this.

This change should not affect any other types of impacts.

patchset2 fixes PEP8 issue

Change-Id: I5c46f0d540949bc4a5de1e57fcd68c537a54ca15
Merge "cease docimpact emails, create bugs only"