rpc, docs: Add note for commands that supports only legacy wallets

Summary:

Note is added for following rpc commands:

importprivkey, importpubkey, importwallet, dumpprivkey,
dumpwallet, importmulti, addmultisigaddress, sethdseed

Backport of core#25680.

Test Plan: Read the doc.

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16576