qt: update request payment button text and tab description

Summary:
Clarify instructions to avoid user confusion.
Backport of Core PR15829

Test Plan: Install, run bitcoin-qt, check new text in widgets.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7776