docs(discovery/service): Clarify multiple service definitions

Be more explicit that the definition of multiple services only works in config files,
not using the HTTP API.

Ref: https://discuss.hashicorp.com/t/register-multiple-services-via-put-request/

website/content/docs/discovery/services.mdx

@@ -27,8 +27,8 @@ To configure a service, either provide the service definition as a
 `-config-file` option to the agent or place it inside the `-config-dir` of the
 agent. The file must end in the `.json` or `.hcl` extension to be loaded by
 Consul. Check definitions can be updated by sending a `SIGHUP` to the agent.
-Alternatively, the service can be registered dynamically using the [HTTP
-API](/api).
+Alternatively, the service can be [registered dynamically](/api-docs/agent/service#register-service)
+using the [HTTP API](/api).
 
 A service definition is a configuration that looks like the following. This
 example shows all possible fields, but note that only a few are required.
@@ -71,7 +71,7 @@ example shows all possible fields, but note that only a few are required.
       "mode": "transparent",
       "transparent_proxy": {
         "outbound_listener_port": 22500
-      }      
+      },
       "config": {},
       "upstreams": [],
       "mesh_gateway": {
@@ -275,8 +275,9 @@ deprecated and has been removed as of Consul 1.1.
 
 ## Multiple Service Definitions
 
-Multiple services definitions can be provided at once using the plural
-`services` key in your configuration file.
+Multiple services definitions can be provided at once when registering services
+via the agent configuration by using the plural `services` key (registering
+multiple services in this manner is not supported using the HTTP API).
 
 ```javascript
 {