Clarify generated setup files

Author: tobiasraabe

docs/source/tutorials/set_up_a_project.md

@@ -14,13 +14,12 @@ move to the next section of the tutorials.
 
 ## The directory structure
 
-The following directory tree gives an overview of the project's different parts after
-the first build.
+The following directory tree gives an overview of the project's different parts.
 
 ```text
 my_project
 │
-├───.pytask
+├───.pytask  # Generated by pytask.
 │
 ├───bld
 │   └────...
@@ -31,7 +30,7 @@ my_project
 │       ├────config.py
 │       └────...
 │
-├───pytask.lock
+├───pytask.lock  # Generated by pytask.
 │
 └───pyproject.toml
 ```
@@ -40,7 +39,7 @@ Create the project files and folders for your project or start from pytask's
 [cookiecutter-pytask-project](https://github.com/pytask-dev/cookiecutter-pytask-project)
 template or any other
 [linked template or example project](../how_to_guides/bp_templates_and_projects.md).
-pytask creates the `.pytask` folder and `pytask.lock` file during builds.
+pytask creates the `.pytask` folder and `pytask.lock` file later when you run tasks.
 
 ## The `src` directory