pgsql: Add missing documentation for argument of amcostestimate()

Add missing documentation for argument of amcostestimate()

5262f7a4fc44 have introduced parallel index scan.  In order to estimate the
number of parallel workers, it adds extra argument to amcostestimate() index
access method API function.  However, this extra argument was missed in the
documentation.  This commit fixes that.

Discussion: https://postgr.es/m/4128fdb4-8b63-2e05-38f6-3125f8c27263%40lab.ntt.co.jp
Author: Tatsuro Yamada, Alexander Korotkov
Backpatch-through: 10

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e01d3477e7486b0bbc6591be67675962218245b2

Modified Files
--------------
doc/src/sgml/indexam.sgml | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)