make machine --user-mode-networking docs more clear

The docs were outdated mentioning the qemu backed for Mac and I find the
way they are written to be a bit confusing.

I think it is best to start with that this option is not supported on
all the providers except WSL.

Fixes: #26780

Signed-off-by: Paul Holzinger <[email protected]>

Author: Luap99

docs/source/markdown/options/user-mode-networking.md

@@ -4,18 +4,21 @@
 ####> are applicable to all of those.
 #### **--user-mode-networking**
 
-Indicates that this machine relays traffic from the guest through a user-space
-process running on the host. In some VPN configurations the VPN may drop
-traffic from alternate network interfaces, including VM network devices. By
-enabling user-mode networking (a setting of `true`), VPNs observe all
-podman machine traffic as coming from the host, bypassing the problem.
+This option can only be used for the WSL provider on Windows. On all other
+platforms this option is ignored and user mode networking will be always be
+`true` there because these providers always depend on gvproxy (our user
+mode networking tool for the VMs)
 
-When the qemu backend is used (Linux, Mac), user-mode networking is
-mandatory and the only allowed value is `true`. In contrast, The Windows/WSL
-backend defaults to `false`, and follows the standard WSL network setup.
+In contrast, The Windows/WSL backend defaults to `false`, and follows the
+standard WSL network setup.
 Changing this setting to `true` on Windows/WSL informs Podman to replace
 the WSL networking setup on start of this machine instance with a user-mode
 networking distribution. Since WSL shares the same kernel across
 distributions, all other running distributions reuses this network.
 Likewise, when the last machine instance with a `true` setting stops, the
 original networking setup is restored.
+
+In some VPN configurations the VPN may drop traffic from alternate network
+interfaces, including VM network devices. By enabling user-mode networking
+VPNs observe all podman machine traffic as coming from the host, bypassing
+the problem.