gnome-calls: document limitations/compatibility

hosts/common/programs/calls.nix

@@ -13,6 +13,18 @@
 # - "Use for Calls" means, "when i click a tel: URI, use this account": <https://gitlab.gnome.org/GNOME/calls/-/issues/513>
 # - `calls -vvv` for verbosity
 # - `SOFIA_DEBUG=9 NEA_DEBUG=9 NUA_DEBUG=9 NTA_DEBUG=9 SU_DEBUG=8 gnome-calls` to debug SIP related stuff
+#
+# LIMITATIONS, COMPATIBILITY (as of 2024-08-20):
+# - when switching from wifi -> wwan (4g), may experience about a minute of audio loss.
+#   the call stays alive, but no sound in either direction.
+#   this appears to be ~40s of general net loss to servo-hn (NetworkManager being slow to switch the default device? wireguard being slow to refresh?),
+#   unknown how much time is lost in the upper layers (e.g. dns being refreshed)
+# - wwan -> wifi switching is (near) flawless. prefer to keep modem powered until end of call, because of audio routing, but OK to power it off.
+# - audio is not always routed to a good device when the modem is powered.
+#   solve by opening `pavucontrol`, go to "configuration" tab, change "Built-in audio" to anything and then back to "Make a phone call (Earpiece, Mic)".
+#   i expect my eg25-control-powered script messes with the audio routing.
+# - `gnome-calls` takes about 2 minutes after launch until it shows the UI.
+#   seems to be sandbox related.
 { config, lib, pkgs, ... }:
 let
   cfg = config.sane.programs.calls;