clarify root privilege requirements

NQPTP works and runs as _shairport user on OpenBSD.

Linux capabilities(7) allow starting as unprivileged user in the first
place, otherwise programs only need to start and perform privileged
actions such as bind(2)ing sockets as root, after that they can and
should! drop privileges.

Instead of updating/repeating supported systems, drop listings.

AIRPLAY2.md

@@ -18,20 +18,20 @@ Shairport Sync offers AirPlay 2 support for audio sources on iOS devices, Macs f
 
 ## General
 Shairport Sync uses a companion application called [NQPTP](https://github.com/mikebrady/nqptp) ("Not Quite PTP")
-for timing and synchronisation in AirPlay 2. NQPTP must run as `root` and must have exclusive access to ports `319` and `320`.
+for timing and synchronisation in AirPlay 2. NQPTP must have exclusive access to ports `319` and `320`.
 
 Lossless and High Definition Lossless material is transcoded to AAC before it reaches Shairport Sync. 
 
 ## What You Need
 AirPlay 2 support needs a slightly more powerful CPU for decoding and synchronisation and more memory for bigger buffers and larger libraries. A system with the power of a Raspberry Pi 2 or Raspberry Pi Zero 2 W, or better, is recommended.
 
 Here are some guidelines: 
-* Full access, including `root` privileges, to a system at least as powerful as a Raspberry Pi 2 or a Raspberry Pi Zero 2 W.
+* Full access, including network capabilities or `root` privileges, to a system at least as powerful as a Raspberry Pi 2 or a Raspberry Pi Zero 2 W.
 * Ports 319 and 320 must be free to use (i.e. they must not be in use by another service such as a PTP service) and must not be blocked by a firewall.
-* An up-to-date Linux or FreeBSD. This is important, as some of the libraries must be the latest available.
+* An up-to-date system. This is important, as some of the libraries must be the latest available.
 * Shairport Sync will not run in AirPlay 2 mode on a Mac because NQPTP, on which it relies, needs ports 319 and 320, which are already used by macOS.
 * A version of the [FFmpeg](https://www.ffmpeg.org) library with an AAC decoder capable of decoding Floating Planar -- `fltp` -- material. There is a guide [here](TROUBLESHOOTING.md#aac-decoder-issues-airplay-2-only) to help you find out if your system has it.
-* An audio output, for example an ALSA device (or `sndio` in FreeBSD). The device must be capable of running at 44,100 frames per second. You can use [`sps-alsa-explore`](https://github.com/mikebrady/sps-alsa-explore) to test the suitability of hardware ALSA audio devices on your device.
+* An audio output. The device must be capable of running at 44,100 frames per second. You can use [`sps-alsa-explore`](https://github.com/mikebrady/sps-alsa-explore) to test the suitability of hardware ALSA audio devices on your device.
 Other backends continue to work as with "classic" Shairport Sync.
 - Multiple instances of the AirPlay 2 version of Shairport Sync can not be hosted on the same system. It seems that AirPlay 2 clients are confused by having multiple AirPlay 2 players at the same IP addresses.