Update documentation for listening to 0.0.0.0 (fastify#5988)

Author: jonasongg

docs/Guides/Getting-Started.md

@@ -128,6 +128,9 @@ above, and more!
 >
 > When deploying to a Docker (or another type of) container using `0.0.0.0` or
 > `::` would be the easiest method for exposing the application.
+>
+> Note that when using `0.0.0.0`, the address provided in the callback argument
+> above will be the first address the wildcard refers to.
 
 ### Your first plugin
 <a id="first-plugin"></a>

docs/Reference/Server.md

@@ -973,7 +973,8 @@ server.listen({
 By default, the server will listen on the address(es) resolved by `localhost`
 when no specific host is provided. If listening on any available interface is
 desired, then specifying `0.0.0.0` for the address will listen on all IPv4
-addresses. The following table details the possible values for `host` when
+addresses. The address argument provided above will then return the first such
+IPv4 address. The following table details the possible values for `host` when
 targeting `localhost`, and what the result of those values for `host` will be.
 
  Host          | IPv4 | IPv6