tweak documentation of report's sections attribute

[wimglenn]

fix some formatting in the TestReport / CollectReport, and reword the description about the 'sections' attribute

this is what it looks like currently: https://docs.pytest.org/en/stable/reference.html#pytest.reports.TestReport.sections

The CollectReport one maybe didn't render at all due to bunk formatting (?)

• "which needs to marshallable" does not make a lot of sense (presumably means json serializable?) but is unnecessary anyway since the type is already annotated as List[Tuple[str, str]] which will be marshallable. Also I think I just now got the joke of why marshmallow project is named so... 😄
• (str, str) just duplicated the existing type annotation so I changed it to (heading, content) to better indicate how each string in the pair is actually used
• also mentioned that the sections is now also used for caplog

[nicoddemus]

Thanks @wimglenn! It does look better. 👍