small addition to dev guide (#595)

Author: mmahsereci

docs/source/development/pull_request.md

@@ -237,6 +237,12 @@ Alternatively, you can run a single specific environment through `tox -e <env>`.
   $ tox -e docs
   ```
 
+- **Additional arguments passed to pytest via tox:** e.g., only run `doctests` in `linalg`.
+
+  ```shell
+  $ tox -e py3 -- src/probnum/linalg
+  ```
+
 Code coverage of the tests is reported via [codecov](https://codecov.io/github/probabilistic-numerics/probnum?branch=main).