doc: certificate hot-reload for xDS gRPC connection (#10628)

Adds documentation for #10163.

Signed-off-by: Tero Saarni <tero.saarni@est.tech>

Mirrored from https://github.com/envoyproxy/envoy @ f5d2cc673786cb40af8e06658b652de037f455a6

envoy/api/v2/core/config_source.proto

@@ -135,6 +135,8 @@ message ConfigSource {
     option (validate.required) = true;
 
     // Path on the filesystem to source and watch for configuration updates.
+    // When sourcing configuration for :ref:`secret <envoy_api_msg_auth.Secret>`,
+    // the certificate and key files are also watched for updates.
     //
     // .. note::
     //

envoy/config/core/v3/config_source.proto

@@ -144,6 +144,8 @@ message ConfigSource {
     option (validate.required) = true;
 
     // Path on the filesystem to source and watch for configuration updates.
+    // When sourcing configuration for :ref:`secret <envoy_api_msg_extensions.transport_sockets.tls.v3.Secret>`,
+    // the certificate and key files are also watched for updates.
     //
     // .. note::
     //