clarify author guide reg changes

[maelle]

Not doing changes during the reviews. cc @yabellini

[maelle]

We want reviewers to review:

• something that isn't changing (except, say, bug fixes especially if the bugs prevent the review)
• the latest / best version of the package according to the author

[maelle]

I thought this was in the author guide but apparently not. 😅

[yabellini]

Thank you!