(#18468) Clarify message in defaults for debian

The comment that tried to explain the START property did not clearly align
with the message generated by the installation:

  Setting up puppet (3.1.0-0.1rc1puppetlabs1) ...
 Starting puppet agent
 puppet not configured to start, please edit /etc/default/puppet to
 enable
 .

This changes the comment in the file to include the word "Enable" so that a
user can more confidently match it up to the "enable" word that shows up in
the message. It also explains the effect of the different values.