Doc: improve description of window function processing.

Doc: improve description of window function processing.

The previous wording talked about a "single pass over the data",
which can be read as promising more than intended (to wit, that only
one WindowAgg plan node will be used).  What we promise is only what
the SQL spec requires, namely that the data not get re-sorted between
window functions with compatible PARTITION BY/ORDER BY clauses.
Adjust the wording in hopes of making this clearer.

Reported-by: Christopher Inokuchi <cinokuchi@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/CABde6B5va2wMsnM79u_x=n9KUgfKQje_pbLROEBmA9Ru5XWidw@mail.gmail.com
Backpatch-through: 13