docs: Use arguments instead of environment variables for remote URL templating

Arguments make more sense for remote URL configuration since clients are
configuring connection parameters, not running processes with environment
variables. The valueHint 'tenant_host' maps to {tenant_host} in the URL.

Author: tadasant

docs/reference/server-json/generic-server-json.md

@@ -683,9 +683,10 @@ This example demonstrates URL templating for remote servers, useful for multi-te
     {
       "type": "streamable-http",
       "url": "https://{tenant_host}/api/mcp",
-      "environmentVariables": [
+      "arguments": [
         {
-          "name": "TENANT_HOST",
+          "type": "positional",
+          "valueHint": "tenant_host",
           "description": "Tenant-specific hostname (e.g., 'us-cell1.example.com')",
           "isRequired": true
         }
@@ -695,7 +696,7 @@ This example demonstrates URL templating for remote servers, useful for multi-te
 }
 ```
 
-Clients can set `TENANT_HOST=us-cell1.example.com` to connect to US Cell 1, or `TENANT_HOST=emea.example.com` to connect to the EMEA deployment. The `{tenant_host}` variable in the URL will be replaced with the environment variable value.
+Clients would configure the tenant host value, and the `{tenant_host}` variable in the URL gets replaced with the provided argument value to connect to the appropriate deployment (e.g., `us-cell1.example.com` or `emea.example.com`).
 
 ### Deprecated Server Example