review the ENC documentation

 * change the section title to the long form so it is more readable
   from the table of contents (TOC), even if you don't know the
   acronym (which is still included)

 * start by more explicitely saying what we use the ENC for, not how
   it works (which we still keep)

 * try to use the active form more (e.g. "Puppet pulls elements"
   instead of "It can be used by Puppet to define elements")

 * use `code` style for variables

 * be more explicit about role::foo

 * use the more commonly use `tor-puppet.git` root instead of
   `/etc/puppet`, because the latter is not explicit, and might hint
   that users should edit it directly on the server (which is
   definitely not the case)