Polish documentation, particularly Stopwatch

[nipafx]

Proposed commit message:

Polish documentation and code, particularly Stopwatch (#461 / #643)

The stopwatch extension had meager documentation, which this change
rectifies. It also improves its production, test, and demo code.

Further changes:

* make sure all extensions mention their thread-safety
* make sure `@Test` is always the first annotation
* remove double empty line before h2 headers
  (because nobody except me cares)
* remove "Introduction" header
  (it's pointless - the first paragraphs are always in intro)
* replace "--" with "-"
  (the former is correct, but harder to stick to)
* a few even smaller improvements

Closes: #461
PR: 643

---

PR checklist

The following checklist shall help the PR's author, the reviewers and maintainers to ensure the quality of this project.
It is based on our contributors guidelines, especially the "writing code" section.
It shall help to check for completion of the listed points.
If a point does not apply to the given PR's changes, the corresponding entry can be simply marked as done.

Documentation (general)

• There is documentation (Javadoc and site documentation; added or updated)
• There is implementation information to describe why a non-obvious source code / solution got implemented
• Site documentation has its own .adoc file in the docs folder, e.g. docs/report-entries.adoc
• Site documentation in .adoc file references demo in src/demo/java instead of containing code blocks as text
• Only one sentence per line (especially in .adoc files)
• Javadoc uses formal style, while sites documentation may use informal style

Documentation (new extension)

• The docs/docs-nav.yml navigation has an entry for the new extension
• The package-info.java contains information about the new extension

Code

• Code adheres to code style, naming conventions etc.
• Successful tests cover all changes
• There are checks which validate correct / false usage / configuration of a functionality and there are tests to verify those checks
• Tests use AssertJ or our own PioneerAssert (which are based on AssertJ)

Contributing

• A prepared commit message exists
• The list of contributions inside README.md mentions the new contribution (real name optional)

[nipafx]

Otherwise all good?

[nipafx]

If so, feel free to merge.

[Michael1993]

Otherwise good.