Docs: Suggestion: Make the need for <?php on error docs pages more obvious

[ThomasLandauer]

On pages like https://psalm.dev/docs/running_psalm/issues/PossiblyNullArgument/, there needs to be a code block containing the <?php tag, and it needs to emit the respective error.
This is what I learnt at #8135 (comment)

While such a check is certainly a good thing, it could be made more obvious for docs contributors.

Idea: Change the tag to:

<?php // code for NameOfTheTestThatAssertsIt, see https://...

... linking to some docs page that explains it. And make the parser (that generates the final HTML page) remove it.

[AndrolGenhald]

Why not just add a comment to the docblock for the test? The test itself is the first place you should look when you notice it's failing.