Update per-role proxy docs

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

Author: djmb

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
+# ```