Update per-role proxy docs

Clarify that proxy: true/proxy: false only belong in the role config,
not at the root level.

lib/kamal/configuration/docs/proxy.yml

@@ -10,11 +10,6 @@
 # They are application-specific, so they are not shared when multiple applications
 # run on the same proxy.
 #
-# The proxy is enabled by default on the primary role but can be disabled by
-# setting `proxy: false`.
-#
-# It is disabled by default on all other roles but can be enabled by setting
-# `proxy: true` or providing a proxy configuration.
 proxy:
 
   # Hosts
@@ -113,3 +108,30 @@ proxy:
     response_headers:
       - X-Request-ID
       - X-Request-Start
+
+# Enabling/disabling the proxy on roles
+#
+# The proxy is enabled by default on the primary role but can be disabled by
+# setting `proxy: false` in the primary role's configuration.
+#
+# ```yaml
+# servers:
+#   web:
+#     hosts:
+#      - ...
+#     proxy: false
+# ```
+#
+# It is disabled by default on all other roles but can be enabled by setting
+# `proxy: true` or providing a proxy configuration for that role.
+#
+# ```yaml
+# servers:
+#   web:
+#     hosts:
+#      - ...
+#   web2:
+#     hosts:
+#      - ...
+#     proxy: true
+# ```