Update and clarify documentation on /etc/finit.d/{available,enabled}

[troglobit]

Issue #335 brought to light how unclear the documentation is on how /etc/finit.d/{available,enabled} works.

• /etc/finit.d/*.conf:
  Purely for static services, similar to if /etc/finit.conf was just split over several files
• /etc/finit.d/available/*.conf:
  Store of available service configuration files, similar to Apache httpd
• /etc/finit.d/enabled/*.conf:
  Symlinks back to ../available/foo.conf to enable foo service