improve "network prune" output to mention custom networks only

The `docker network prune` command removes unused custom networks,
but built-in networks won't be removed. This patch updates the
message to mention that it's only removing custom networks.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit eb93a865ed8be7b28e4b03eba975b242d231940d)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: c38b2600770b9199913f0b6065da5a2f5ddd7b0b
Component: cli

Author: thaJeztah

components/cli/cli/command/network/prune.go

@@ -43,7 +43,7 @@ func NewPruneCommand(dockerCli command.Cli) *cobra.Command {
 	return cmd
 }
 
-const warning = `WARNING! This will remove all networks not used by at least one container.
+const warning = `WARNING! This will remove all custom networks not used by at least one container.
 Are you sure you want to continue?`
 
 func runPrune(dockerCli command.Cli, options pruneOptions) (output string, err error) {

components/cli/docs/reference/commandline/network_prune.md

@@ -27,7 +27,7 @@ by any containers.
 ```bash
 $ docker network prune
 
-WARNING! This will remove all networks not used by at least one container.
+WARNING! This will remove all custom networks not used by at least one container.
 Are you sure you want to continue? [y/N] y
 Deleted Networks:
 n1