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
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/26853a86ae4bc4f214005e05f55677e5b228f4f8

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