Clarify instructions for firewall setup
Clarify instructions for firewall setup

* Removes reference to CIDR routing prefix defaulting to `/24` when
  setting up an interface for the first time; it defaults to `/32`, and
  pfSense does not attempt to figure this out for you. Leaves the
  reminder to double-check the CIDR prefix since that has caused
  problems in the past.
* Makes sure text quoted from the pfSense WebGUI matches the text in the
  latest version.
* Various wording changes for readability and clarity.

micahflee/securedrop,conorsch/securedrop,ageis/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop,kelcecil/securedrop,ehartsuyker/securedrop,kelcecil/securedrop,micahflee/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop,ageis/securedrop,ehartsuyker/securedrop,micahflee/securedrop,heartsucker/securedrop,heartsucker/securedrop,heartsucker/securedrop,ageis/securedrop,pwplus/securedrop,pwplus/securedrop,pwplus/securedrop,micahflee/securedrop,pwplus/securedrop,kelcecil/securedrop,kelcecil/securedrop,ageis/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,pwplus/securedrop,kelcecil/securedrop,kelcecil/securedrop,conorsch/securedrop,pwplus/securedrop,conorsch/securedrop,garrettr/securedrop,garrettr/securedrop