Junit5: Improve documentation for writing tests and contional test execution

[aepfli]

During the implementation of #2016 we could see that crucial information regarding conditional test execution was missing within the documentation.

Although the information is present in the Extensions section of the User Guide ...

When multiple ExecutionCondition extensions are registered, a container or test is
disabled as soon as one of the conditions returns _disabled_. Thus, there is no guarantee
that a condition is evaluated because another extension might have already caused a
container or test to be disabled. In other words, the evaluation works like the
short-circuiting boolean OR operator.

... this kind of information is missing within the Writing Tests section, which can lead to confusion when people start using multiple ExecutionConditions.

Deliverables

• [x] Update User Guide to present or link crucial information

[aepfli]

there is already a link in place, which might be overseen, when just looking at the docs. i think that maybe adding just the first sentence to the "writing-docs" documentation, might be useful for developers. As a link might be easily over looked.

[marcphilipp]

I agree that adding the first sentence to the end of the first paragraph makes sense. 👍

[aepfli]

Perfect than I will open the pr tomorrow ;)

[sormuras]

Thanks, @aepfli!