DOC: Point out that inverse modeling needs an average reference projector ready to not raise an error

[nabilalibou]

Hello,

Some documentation enhancement discussed in #12401

[welcome]

Hello! 👋 Thanks for opening your first pull request here! ❤️ We will try to get back to you soon. 🚴

[drammock]

Text changes look good, but the rendering of the final paragraph is messed up (it seems that italicizing the whole paragraph prevents the preformatting and cross-references from being handled). See here:

https://output.circle-artifacts.com/output/job/1e925b38-f8f2-44c7-9c40-c2c1df117da7/artifacts/0/html/auto_tutorials/preprocessing/55_setting_eeg_reference.html#eeg-reference-and-source-modeling

One fix is to just remove the * at beginning and end that italicizes the whole paragraph. Another option is to convert the whole paragraph into an admonition. I kinda like the admonition approach; they were designed to call attention to things, and do a better job of that than just italics. It would be something like this:

# .. important::
#     For the above reasons, when performing inverse imaging, MNE-Python will raise
#     a ``ValueError`` if there are EEG channels present and something other than
#     an average reference projector strategy has been specified. To ensure
#     correct functioning consider calling
#     :meth:`~mne.io.Raw.set_eeg_reference(projection=True)` to add an average
#     reference projector*.

Final note: you either need to add a changelog entry (doc/changes/devel/12420.bugfix.rst, see other files in that folder for format/inspiration), or add a PR label no-changelog-entry-needed.

[larsoner]

Just two suggested tweaks to make the RST render more nicely, otherwise LGTM

[drammock]

approved pending a good-looking HTML render on CircleCI. 🤞🏻

[drammock]

oops, one more fix needed: the changelog filename should have this PR number (12420) not the number of the issue it fixes (12401). See https://github.com/mne-tools/mne-python/actions/runs/7803187571/job/21282378841?pr=12420#step:3:16

[larsoner]

@drammock GitHub UI allowed me to change the name, so I took care of that part!

[welcome]

🎉 Congrats on merging your first pull request! 🥳 Looking forward to seeing more from you in the future! 💪