Clarify documentation on how to build a job runner

[jdavcs]

This is a very inconsequential edit. Nevertheless, I think it may prevent some confusion that may result when reading the tutorial (see this discussion). Without this edit, it's easy to interpret the description of what the parent class does as a guide to what should be implemented in the new subclass.

UPDATE: In response to reviewer comments, I've provided a clearer explanation of what to override and where. While doing that, I've also made some minor edits to formatting and language which, I hope, improve the tutorial.

Also, fixes #16671.

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
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[dannon]

Can you add a bit more detail here? Maybe just another sentence explaining that the parent class handles setup and teardown, and you should only implement the extended logic. As it is I don't think this clarifies it quite enough.

[jdavcs]

I've edited the wording. Apparently, all but the __init__ method should be indeed implemented from start to finish, whereas __init__ may be (and usually is) overwritten. I think this new wording makes it more straightforward. (Thanks for suggesting this edit!)

I've also fixed some formatting issues (list nesting, section and subsection titles) that have been bugging me ever since I read this document 5 years ago, but which I never got to addressing.. The language edits are all minor; please feel free to push to my branch if you have better suggestions.

[mvdbeek]

Nice improvement, thank you!