Remove incorrect difference between user-defined and default bridge (docker#10320)

* Remove incorrect difference between user-defined and default bridge

This PR removes the first bullet point from the list differences between user-defined bridge networks and the default bridge network:

[User-defined bridges provide better isolation and interoperability between containerized applications](https://docs.docker.com/network/bridge/#differences-between-user-defined-bridges-and-the-default-bridge).

The default bridge network, similar to user-defined bridges, opens all ports to other containers and allows for inter-container communication by default. This is elaborated and demonstrated in docker#8973. Other relevant issue: docker#8437.

* Remove mention of previous bullet point

* Added isolation advantage of user-defined over default network

Author: Maximization

network/bridge.md

@@ -34,36 +34,16 @@ network.**
 
 ## Differences between user-defined bridges and the default bridge
 
-- **User-defined bridges provide better isolation and interoperability between containerized applications**.
-
-  Containers connected to the same user-defined bridge network automatically
-  expose **all ports** to each other, and **no ports** to the outside world. This allows
-  containerized applications to communicate with each other easily, without
-  accidentally opening access to the outside world.
-
-  Imagine an application with a web front-end and a database back-end. The
-  outside world needs access to the web front-end (perhaps on port 80), but only
-  the back-end itself needs access to the database host and port. Using a
-  user-defined bridge, only the web port needs to be opened, and the database
-  application doesn't need any ports open, since the web front-end can reach it
-  over the user-defined bridge.
-
-  If you run the same application stack on the default bridge network, you need
-  to open both the web port and the database port, using the `-p` or `--publish`
-  flag for each. This means the Docker host needs to block access to the
-  database port by other means.
-
 - **User-defined bridges provide automatic DNS resolution between containers**.
 
   Containers on the default bridge network can only access each other by IP
   addresses, unless you use the [`--link` option](/network/links/), which is
   considered legacy. On a user-defined bridge network, containers can resolve
   each other by name or alias.
 
-  Imagine the same application as in the previous point, with a web front-end
-  and a database back-end. If you call your containers `web` and `db`, the web
-  container can connect to the db container at `db`, no matter which Docker host
-  the application stack is running on.
+  Imagine an application with a web front-end and a database back-end. If you call
+  your containers `web` and `db`, the web container can connect to the db container
+  at `db`, no matter which Docker host the application stack is running on.
 
   If you run the same application stack on the default bridge network, you need
   to manually create links between the containers (using the legacy `--link`
@@ -72,6 +52,12 @@ network.**
   Alternatively, you can manipulate the `/etc/hosts` files within the containers,
   but this creates problems that are difficult to debug.
 
+- **User-defined bridges provide better isolation**.
+
+  All containers without a `--network` specified, are attached to the default bridge network. This can be a risk, as unrelated stacks/services/containers are then able to communicate.
+
+  Using a user-defined network provides a scoped network in which only containers attached to that network are able to communicate.
+
 - **Containers can be attached and detached from user-defined networks on the fly**.
 
   During a container's lifetime, you can connect or disconnect it from