Add docstring detailing factory use case for ConfigurableResource (#1…

…7613)

## Summary & Motivation

Per #17551 adding docblock description of using `ConfigurableResource` to produce different object type at runtime.

## How I Tested These Changes

![Screenshot 2023-11-01 at 4 52 02 PM](https://github.com/dagster-io/dagster/assets/28738937/06d1b84e-00cc-42b3-8c87-daaeae89b758)

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python_modules/dagster/dagster/_config/pythonic_config/resource.py

@@ -677,6 +677,34 @@ def asset_that_uses_writer(writer: WriterResource):
             resources={"writer": WriterResource(prefix="a_prefix")},
         )
 
+    You can optionally use this class to model configuration only and vend an object
+    of a different type for use at runtime. This is useful for those who wish to
+    have a separate object that manages configuration and a separate object at runtime. Or
+    where you want to directly use a third-party class that you do not control.
+
+    To do this you override the `create_resource` methods to return a different object.
+
+    .. code-block:: python
+
+        class WriterResource(ConfigurableResource):
+            str: prefix
+
+            def create_resource(self, context: InitResourceContext) -> Writer:
+                # Writer is pre-existing class defined else
+                return Writer(self.prefix)
+
+    Example usage:
+
+    .. code-block:: python
+
+        @asset
+        def use_preexisting_writer_as_resource(writer: ResourceParam[Writer]):
+            writer.output("text")
+
+        defs = Definitions(
+            assets=[use_preexisting_writer_as_resource],
+            resources={"writer": WriterResource(prefix="a_prefix")},
+        )
     """
 
     def create_resource(self, context: InitResourceContext) -> TResValue: