[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