Beautify some docs properties

There are some things in the auto-generated docs website that could be improved.

1. Sometimes, the listed attributes are enclosed in double backticks (e.g. https://martinos.org/mne/dev/generated/mne.preprocessing.Xdawn.html#mne.preprocessing.Xdawn, https://martinos.org/mne/dev/generated/mne.decoding.CSP.html#mne.decoding.CSP). These double backticks get rendered in the docs, which is probably not what we want.
2. If there is a references section, each listed reference is preceded by (1, 2) which should point to all occurrences in the text. However, the 2 entry is there even though a reference is only mentioned once in the text, and thus the link doesn’t work.
3. I wonder if it’s correct to use double backticks when referring to arguments in the description (see e.g. the CSP docs, where the reg parameter is referred to in the description). In the type description of the parameters, these are usually rendered italic, so maybe we should enclose only with single backticks to be consistent.

[larsoner]

Yes, any other choice would require a discussion 😃

[larsoner]

So this could already be fixed?

Yes it’s in #5120