docs: Write to secrets store is not supported by design (apache#37814)

* docs: Write to secrets store is not supported by design

* fix build docs

* small clarification

docs/apache-airflow-providers/core-extensions/secrets-backends.rst

@@ -26,6 +26,24 @@ than from its own Database. While storing such information in Airflow's database
 enterprise customers already have some secret managers storing secrets, and Airflow can tap into those
 via providers that implement secrets backends for services Airflow integrates with.
 
+.. note::
+
+  Secret Backend integration do not allow writes to the secret backend.
+  This is a design choice as normally secret stores require elevated permissions to write as it is a protected resource.
+  That means ``Variable.set(...)`` will write to the Airflow metastore even if you use secret backend.
+  If you need to update a value of a secret stored in the secret backend you must do it explicitly. That can be done
+  by using operator that writes to the secret backend of your choice.
+
+.. warning::
+
+  If you have key ``foo`` in secret backend and you will do ``Variable.set(key='foo',...)`` it will create
+  Airflow Variable with key ``foo`` in the Airflow metastore. It means you will have 2 secrets with key ``foo``.
+  While this is possible, Airflow detects that this situation is likely wrong and output to the task log a warning that
+  explains while the write request is honored it will be ignored with the next read. The reason for this is when
+  executing ``Variable.get('foo')``, it will read the value from the secret backend. The value stored in Airflow
+  metastore will be ignored due to priority given to the secret backend.
+
+
 You can also take a
 look at Secret backends available in the core Airflow in
 :doc:`apache-airflow:security/secrets/secrets-backend/index` and here you can see the ones