[backport#18777] wallet: Recommend absolute path for dumpwallet

Summary:
fa501700e91b8667d4d2f116c3705e3ab9a1c8c3 wallet: Recommned absolute path for dumpwallet (MarcoFalke)

Pull request description:

Avoids misunderstandings such as #9564

Backport of Core PR18777

Test Plan:

ninja all
./src/bitcoin-cli help dumpwallet

Check the help output is correct

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8994