doc: clarify ica.apply include and exclude params

[sappelhoff]

That's how I understand it (and the code looks like it in the source and when running it) ... so I thought I'd add this to the docstring to clear things up.

The relevant things happen here:

mne-python/mne/preprocessing/ica.py

Lines 1796 to 1801 in c3ff05c

	def _pick_sources(self, data, include, exclude, n_pca_components):
	"""Aux function."""
	if n_pca_components is None:
	n_pca_components = self.n_pca_components
	data = self._pre_whiten(data)
	exclude = self._check_exclude(exclude)

where _check_exclude does this (either union of ica.exclude and exclude, or just ica.exclude, if exclude is "n/a"):

mne-python/mne/preprocessing/ica.py

Lines 1726 to 1731 in c3ff05c

	def _check_exclude(self, exclude):
	if exclude is None:
	return list(set(self.exclude))
	else:
	# Allow both self.exclude and exclude to be array-like:
	return list(set(self.exclude).union(set(exclude)))

include is handled a few lines below in _pick_sources:

mne-python/mne/preprocessing/ica.py

Lines 1820 to 1824 in c3ff05c

	sel_keep = np.arange(self.n_components_)
	if include not in (None, []):
	sel_keep = np.unique(include)
	elif exclude not in (None, []):
	sel_keep = np.setdiff1d(np.arange(self.n_components_), exclude)

i.e., all are included if None ... else the specific selection specified ... and then idxs are removed from it using exclude as previously triaged