message: Improve clarity of systemd service file comments
  The phrasing seemed to indicate that the options specified in
  ExecStart= could not be specified in the config file, necessitating
  their inclusion in the service file. However, the options in the
  config file simply get overridden by any specified in ExecStart=.
diff: # See "man systemd.service" for details.
  # Note that almost all daemon options could be specified in
  -# /etc/bitcoin/bitcoin.conf, except for those explicitly specified as arguments
  -# in ExecStart=
  +# /etc/bitcoin/bitcoin.conf, but keep in mind those explicitly
  +# specified as arguments in ExecStart= will override those in the
  +# config file.
  [Unit]
  Description=Bitcoin daemon
  