Some warnings and an example for clarity (opnsense#444)

* Some warnings and an example for clarity 

Added a warning about overlapping networks for the LAN and Wireguard interfaces, as doing so will cause the clients not to be able to communicate with each other properly. Added an example to request another PD from your ISP, as ATT only hands out /64s and one will not be enough to subnet the networks properly.

* Clarification and briefness

Tried to add just enough information to clarify that it is a completely separated network that requires routing to reach the LAN interface.

Author: TeamLinux01

source/manual/how-tos/wireguard-client.rst

@@ -38,7 +38,7 @@ Step 2 - Configure the local peer (server)
 
 .. Note::
 
-    The tunnel address must be in CIDR notation and must be a unique IP and subnet for your network. The subnet should be an appropriate size that includes all the client peers that will use the tunnel. For IPv4 it should be a private (RFC1918) address, for example 10.10.10.1/24. For IPv6, it could either be a unique ULA /64 address, or a unique GUA /64 address derived from your prefix delegation. **Do not use a tunnel address that is a /32 (IPv4) or a /128 (IPv6)**
+    The tunnel address must be in CIDR notation and must be a unique IP and subnet for your network, such as if it was on a physically different routed interface. The subnet should be an appropriate size that includes all the client peers that will use the tunnel. For IPv4 it should be a private (RFC1918) address, for example 10.10.10.1/24. For IPv6, it could either be a unique ULA /64 address, or a unique GUA /64 address derived from your prefix delegation. **Do not use a tunnel address that is a /32 (IPv4) or a /128 (IPv6)**
 
 .. Note::