gui: Added label & tooltip for Verify Message labels

Summary:
This serves as documentation for the Verify Message tool.

This is a backport of Core PR17125

Test Plan:
ninja && src/qt/bitcoin-qt

Menu File -> Verify Message

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8160