Expand directive format validation in doc building

[tsbinns]

A while back checks were added to the docs build procedure that malformed sphinx directives would be caught: https://github.com/mne-tools/mne-python/blob/main/doc/sphinxext/directive_formatting.py
However, the logic isn't exhaustive.

A warning directive is present in the Notes section of apply_hilbert, but it is missing a second colon:

mne-python/mne/filter.py

Lines 2747 to 2751 in 5e17532

	.. warning: Do not use ``envelope=True`` if you intend to compute
	an inverse solution from the raw data. If you want to
	compute the envelope in source space, use
	``envelope=False`` and compute the envelope after the
	inverse solution has been obtained.

As such, this never appears:

We should extend the logic of the checks to catch cases like this.