Update "custom markers" example in reference to use new marker API

[nicoddemus]

Just noticed that the section custom markers shows an example using Node.get_marker. We should update the example in the features branch to showcase the new markers API, possibly showing a case where markers are applied multiple times (for example in a class and a test function at the same time).

cc @RonnyPfannschmidt

[pytestbot]

GitMate.io thinks possibly related issues are #2153 (Bad user experience using custom markers on params), #631 (markers leaking), #2999 (Update public api for getting finalizers), #277 (Markers as functions), and #379 (Can't not apply multiple markers to individual test when using parametrize).

[Lekensteyn]

Hasn't this issue been resolved already via https://docs.pytest.org/en/latest/mark.html?
The linked section in the reference also mentions the new Node.iter_markers API which links to said page.

[nicoddemus]

Thanks @Lekensteyn, this issue was already fixed then. 👍