De-obfuscate the comment in tsrank.c's calc_rank_or().

De-obfuscate the comment in tsrank.c's calc_rank_or().

Oleg's original comment was intelligible only to him.
Aleksander has reverse-engineered what seems like a plausible
explanation of what the code is trying to do, so replace the
comment with that.  (Also, re-order the final expression to
match the new comment.)

In passing, this makes the comment satisfy our usual formatting
conventions.  pgindent has let it pass as-is so far, but planned
changes would mess it up without some sort of intervention.

Author: Aleksander Alekseev <aleksander@tigerdata.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAJ7c6TO0xvunpeOv89i1eKQBhKF9=GEETkTz+yAGs1xGYH25MQ@mail.gmail.com