Add section to howtos.md (#14171)

docs/source/contributor-guide/howtos.md

@@ -146,3 +146,15 @@ valid installation of [protoc] (see [installation instructions] for details).
 
 [protoc]: https://github.com/protocolbuffers/protobuf#protocol-compiler-installation
 [installation instructions]: https://datafusion.apache.org/contributor-guide/getting_started.html#protoc-installation
+
+## How to add/edit documentation for UDFs
+
+Documentations for the UDF documentations are generated from code (related [github issue]). To generate markdown run `./update_function_docs.sh`.
+
+This is necessary after adding new UDF implementation or modifying existing implementation which requires to update documentation.
+
+```bash
+./dev/update_function_docs.sh
+```
+
+[github issue]: https://github.com/apache/datafusion/issues/12740