Document pytest.param

[dawranliou]

Append example for pytest.param in the example/parametrize document.

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[dawranliou]

This PR is targeting #2658 . Any suggestion is welcome! Thanks!

[coveralls]

Coverage remained the same at 92.244% when pulling af5e923 on dawran6:2658 into 111d640 on pytest-dev:master.

[RonnyPfannschmidt]

as far as i can tell the ci errors are unrelated,

thanks 👍