DOC: Make compute_raw_covariance easier to find (#7533)
DOC: Make compute_raw_covariance easier to find (#7533)

* DOC: Make compute_raw_covariance easier to find

A short support request from @virvw made it obvious to me that
`mne.compute_raw_covariance` is currently not so easy to discover
as it should be; users were trying to use `mne.compute_covariance`
for empty-room data (and, consequently, running into trouble),
without realizing that `compute_raw_covariance` was readily
available. This commit adds a clearly-visible note to the docstring
to help guide users better.

* DOC: Extend See Also section

* Update docstring of compute_raw_covariance too
olafhauk/mne-python,larsoner/mne-python,mne-tools/mne-python,kingjr/mne-python,wmvanvliet/mne-python,cjayb/mne-python,drammock/mne-python,Teekuningas/mne-python,drammock/mne-python,Eric89GXL/mne-python,rkmaddox/mne-python,wmvanvliet/mne-python,kingjr/mne-python,Teekuningas/mne-python,mne-tools/mne-python,pravsripad/mne-python,kambysese/mne-python,larsoner/mne-python,olafhauk/mne-python,mne-tools/mne-python,drammock/mne-python,bloyl/mne-python,olafhauk/mne-python,kingjr/mne-python,Eric89GXL/mne-python,kambysese/mne-python,bloyl/mne-python,pravsripad/mne-python,Teekuningas/mne-python,wmvanvliet/mne-python,cjayb/mne-python,rkmaddox/mne-python,pravsripad/mne-python,larsoner/mne-python