api: add deprecation warning to comment (#3511)

The proto field is marked as deprecated without any explanation, so this
adds a reference to the other field which should be used instead.

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ 0bcdb5d7611a79fd22f823fd707a8b6f7b5f756e

envoy/config/filter/http/health_check/v2/health_check.proto

@@ -21,6 +21,8 @@ message HealthCheck {
 
   // Specifies the incoming HTTP endpoint that should be considered the
   // health check endpoint. For example */healthcheck*.
+  // Note that this field is deprecated in favor of
+  // :ref:`headers <envoy_api_field_config.filter.http.health_check.v2.HealthCheck.headers>`.
   string endpoint = 2 [deprecated = true];
 
   // If operating in pass through mode, the amount of time in milliseconds