Document that pytest.warns affects default DeprecationWarning processing

[nicoddemus]

As discussed in https://github.com/pytest-dev/pytest/issues/9288#issuecomment-965102239, using pytest.warns will by design also capture DeprecationWarnings which would by default be captured by pytest.

We should add a note about that in pytest.warns and the section about DeprecationWarnings handling.

[toclarity]

Hi @nicoddemus, from @clarityai-eng we'd like to contribute with this PR to make the documentation clearer on how pytest.warn is affecting other posible raised warnings.

We hope you find it useful, let us know if it fits your requirements.

[toclarity]

Hi! We have updated the MR, please take a look at it whenever you can. Thanks! :)