chore(api): clarify the difference between window functions with unspecified windows, unbounded on both sides, and unbounded preceding and current row

[cpcloud]

I added an extensive comment. I think we have some follow up work to do around clarifying the difference between rows=(None, None) and rows=(None, 0).

Originally posted by @cpcloud in #7993 (comment)

[chloeh13q]

Is this just about clarifying this in the documentation for window functions here: https://ibis-project.org/tutorials/ibis-for-sql-users.html#window-functions ?