[backport#15426] [Doc] importmulti: add missing description of keypool option

Summary:
[Doc] importmulti: add missing description of keypool option (David A. Harding)

Pull request description:

Option was added in #14075 but not documented there.

CC: @achow101

https://github.com/bitcoin/bitcoin/pull/15426/commits/a607c9ae4c2730fca5ce340c400c95c87e498a7c

Depends on D7349

Backport of Core PR15426

Test Plan:

ninja
./src/bitcoind -regtest
./src/bitcoin-cli -regtest help importmulti

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7330