Document disabling with '-n 0'

[adamchainz]

I found that -n 0 was not properly documented, just in this mention in the "how to" guide:

pytest-xdist/docs/how-to.rst

Lines 21 to 22 in f83299c

	When ``xdist`` is disabled (running with ``-n0`` for example), then
	``worker_id`` will return ``"master"``.

Plus some error messages and docstrings like:

pytest-xdist/src/xdist/plugin.py

Lines 305 to 308 in 8074c63

	if not val("collectonly") and _is_distribution_mode(config) and usepdb:
	raise pytest.UsageError(
	"--pdb is incompatible with distributing tests; try using -n0 or -nauto."
	)

This PR makes it explicit. I don't think a changelog entry is required.

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