DOC: clarify add_reference_channels usage (Takeover #13664)

[Dpereaptkhamur-13]

Reference issue (if any)

What does this implement/fix?

This PR completes the documentation clarification for add_reference_channels originally started in #13664. It addresses the mathematical requirements for average referencing in the docstrings, as requested in issue #13618.
Changes include:

• Added an explicit note to the add_reference_channels docstring explaining average reference recomputation.
• Included a properly formatted towncrier entry in doc/changes/dev/13618.notable.rst to resolve the check failures seen in the previous PR.

Additional information

[HansujaB]

Hey @Dpereaptkhamur-13 , I believe you don't have an account on circle ci that's why those checks are failing , login into your account & those tests will pass !

[Dpereaptkhamur-13]

Hi @HansujaB, thank you for the tip! I have now logged into CircleCI and authorized the account. The tests should be ready to run once a maintainer approves the workflow. Thanks again for the help!

[tsbinns]

Hi @Dpereaptkhamur-13, thanks for opening the PR.

Right now, the message has been added to add_reference_channels. However, the risk is that people are calling set_eeg_reference without calling add_reference_channels first. So, this note about the proper procedure could be easily missed by the people who need to see it most.

Rather, the note should be included in set_eeg_reference, such that people see this, and then go back to use add_refence_channels. I'd suggest looking at how this was handled in #13664 and adding the note to the docdict: https://github.com/Farzah11/mne-python/blob/a8e92a3a7d8a83a021d81da9c6b1b38f678ad53f/mne/utils/docs.py#L4106-L4112

Just make sure if you use this text to action the suggested changes: https://github.com/mne-tools/mne-python/pull/13664/changes#r2892847499 and https://github.com/mne-tools/mne-python/pull/13664/changes#r2892851172

Then, it would be good to also add a note to the setting_eeg_reference tutorial, similar to in #13664: https://github.com/Farzah11/mne-python/blob/a8e92a3a7d8a83a021d81da9c6b1b38f678ad53f/tutorials/preprocessing/55_setting_eeg_reference.py#L144-L150
However, also keep in mind the formatting suggestion to get this to render properly: https://github.com/mne-tools/mne-python/pull/13664/changes#r2892854083

[Dpereaptkhamur-13]

Hi @tsbinns, thank you for the detailed feedback! That makes complete sense—moving the clarification to set_eeg_reference and the tutorials ensures it's visible to users regardless of which function they check first. I will update mne/utils/docs.py and the preprocessing tutorial file accordingly. I'll ping you again once the changes are pushed!