Document that pytest's log capturing counts toward memory limits

[mkzung]

Fixes #29.

pytest keeps every log record a test emits, and those allocations count toward limit_memory. This test fails with 17.5 MiB allocated and passes with -p no:logging:

import logging

import pytest

log = logging.getLogger(__name__)


@pytest.mark.limit_memory("1 MB")
def test_many_warnings():
    for i in range(20_000):
        log.warning("message %d", i)

This adds a note saying so under limit_memory in the usage docs, with a news fragment.