Fix various query jumble comments

Fix various query jumble comments

Some comments for struct WindowFunc were trying to detail which fields
were irrelevant for query jumble but the list had not been kept
up-to-date.  Here we fix that by removing the comment to allow the
"query_jumble_ignore" attribute to self-document.  This involved
removing similar comments from other structs.

While we're on the topic, improve comments around why Consts only jumble
the "consttype" and also add some rationale about why various other fields
are ignored.

Reported-by: jian he <jian.universality@gmail.com>
Author: David Rowley <dgrowleyml@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CACJufxEWeP2SLVMsbFNynd0pQnwbxh6U-v1nq5ccf9mSvBZntw%40mail.gmail.com