Clarify test method discovery and supported formats in User Guide (CLI)

[rcasia]

Issue: #3748

Overview

This PR adds clarity to the usage of test methods with the --select-method argument for CLI users.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[mpkorstanje]

I think this is a good suggestion.

But I'm not part of the JUnit Team, so I can't say anything with authority. Usually they'll ask for contributions, or if it is small stuff, to avoid the back and forth they'll do it directly.

That said, I think it would help if:

• The formatting in the issue was fixed.
• There was an actual changelog item.

[sbrannen]

If we are going to be more explicit about the syntax for method selectors, we should also be more explicit about other selectors: packages, classes, etc.

For example, those all have to be "fully-qualified".

[sbrannen]

That said, I think it would help if:

* The formatting in the issue was fixed.

* There was an actual changelog item.

I agree with both of those points.

The formatting should align with the existing formatting (line breaks, etc.) in the same file.

This would warrant a minimal entry in the release notes.