update merge_batch's docs for opt_filter in GroupsAccumulator

[haohuaijin]

I am also a bit confused for this API, what's the semantics for opt_filter? In update_filter it's quite obvious, they refer to the filter in the original query like avg(x) filter x>0, here can we assume they're always None, and for groups with Null state, it should be encoded in the values null mask?

I can't find the doc on trait, it seem to depend on implementation now. I suggest we can figure it out the existing practice and update the doc. (in follow up PR for sure)

Originally posted by @2010YOUY01 in #22768 (comment)