Custom function documentation: enable cache, links to examples. (#32)

Author: badmonster0

docs/docs/core/custom_function.mdx

@@ -56,7 +56,7 @@ A function executor is defined as a class annotated by `@cocoindex.op.executor_c
 
 
 ```python
-@cocoindex.op.executor_class()
+@cocoindex.op.executor_class(...)
 class DemoFunctionExecutor:
     spec: DemoFunctionSpec
     ...
@@ -70,12 +70,39 @@ class DemoFunctionExecutor:
 
 Notes:
 
-*   The `cocoindex.op.executor_class()` class also takes the following input:
+*   The `cocoindex.op.executor_class()` class decorator also takes the following optional arguments:
+
     *   `gpu: bool`: Whether the executor will use GPU. It will affect the way the function is scheduled.
+
+    *   `cache: bool`: Whether the executor will enable cache for this function.
+         When `True`, the executor will cache the result of the function for reuse during reprocessing.
+         We recommend to set this to `True` for any function that is computationally intensive.
+
+    *   `behavior_version: int`: The version of the behavior of the function.
+        When the version is changed, the function will be re-executed even if cache is enabled.
+        It's required to be set if `cache` is `True`.
+
+    For example, this enables cache for the function:
+
+    ```python
+    @cocoindex.op.executor_class(cache=True, behavior_version=1)
+    class DemoFunctionExecutor:
+        ...
+    ```
+
 *   A `spec` field must be present in the class, and must be annoated with the spec class name.
 *   The `prepare()` method is optional. It's executed once and only once before any `__call__` execution, to prepare the function execution.
 *   The `__call__()` method is required. It's executed for each specific rows of data.
     Types of arugments and the return value must be annotated, so that CocoIndex will have information about data types of the operation's output fields. See [Data Types](/docs/core/data_types) for supported types.
 
 </TabItem>
-</Tabs>
+</Tabs>
+
+## Examples
+
+The cocoindex repository contains the following examples of custom functions:
+
+*   In the [pdf_embedding](https://github.com/cocoindex-io/cocoindex/blob/main/examples/pdf_embedding/pdf_embedding.py) example, we define a custom function `PdfToMarkdown`
+*   The `SentenceTransformerEmbed` function shipped with the CocoIndex Python package is defined by Python SDK.
+    Search for [`SentenceTransformerEmbedExecutor`](https://github.com/search?q=repo%3Acocoindex-io%2Fcocoindex+SentenceTransformerEmbedExecutor&type=code) to see the code.
+