Merge #12501: [qt] Improved "custom fee" explanation in tooltip

Summary:
0bc095efd [qt] Improved "custom fee" explanation in tooltip (Randolf Richardson)

Pull request description:

Thanks to @dooglus for asking about this tooltip in Issue 12500.
Reference:  https://www.github.com/bitcoin/bitcoin/issues/12500

I would also appreciate it if someone can confirm that 1 kilobyte in this field indeed represents 1,000 bytes rather than 1,024 bytes (if it's supposed to be 1,024, then I'll gladly make the necessary changes to reflect this).

Tree-SHA512: da2fe0128411b5ef6f0a26382a80601efcf823c3f3591bdd83a7fe7e25777728e7eb89e2e8b175b991566e63838aca12d204792f981031b86e7b2ba28ca50021

Backport of Core PR12501
https://github.com/bitcoin/bitcoin/pull/12501/files

Test Plan:

make check
bitcoin-qt
# Hover over the 'per Kilobyte' note to see the updated tooltip

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3206