updated openapi page - optional operationId

Signed-off-by: bidi47 <bidi@apidemia.com>

Author: bidi47

docs/book/v7/openapi/write-documentation.md

@@ -101,6 +101,14 @@ It should specify at least the following parameters:
 - `parameters`: an array of `query`/`path` parameters - each parameter is specified as a new `OA\Parameter` object
 - `responses`: an array of `OA\Response` objects, each describing a combination of HTTP status codes and their respective response bodies
 
+### Optional parameters
+
+The following parameter is available on every request object but is not required:
+
+- `operationId`: a name for the operation, unique across the whole document (example: `getUserCollection`).
+  Client generators typically use it as the method name, and `Link` objects use it to refer to an operation.
+  The endpoints shipped with Dotkernel API do not set it.
+
 ## Conclusion
 
 To summarize, the typical scenario on working on your own instance of Dotkernel API would follow these steps:
@@ -131,6 +139,12 @@ A: `path`, `description`, `summary`, `tags`, `parameters` and `responses`, plus
 
 A: Omit the `security` parameter.
 
+**Q: Do I need `operationId`?**
+
+A: No.
+It is optional and the shipped endpoints omit it.
+Set it if you generate a client from the specification and want stable method names; it must be unique across the document.
+
 **Q: What is the difference between `description` and `summary`?**
 
 A: `summary` is a short one-line label; `description` is the verbose explanation.