rpc: encryptwallet help, mention HD seed rotation and backup requirement

Summary:
Small and simple PR, updating the encryptwallet help message.

Better to notify users about the HD seed rotation and the new
backup requirement before executing the encryption process.
Ensuring they are prepared to update previous backups and
securely safeguard the updated wallet file.

Backport of core#28980

Reviewer note: This was an AI-automated backport.

Test Plan:

ninja && ninja check check-functional
./src/bitcoind -regtest
./src/bitcoin-cli -regtest help encryptwallet

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D20225