gui: Improved tooltip for send amount field
gui: Improved tooltip for send amount field

Summary:
This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/17180 | PR17180]] and [[https://github.com/bitcoin/bitcoin/pull/17226 | PR17226]]

The first PR fixed the message but on the wrong widget. The second PR sets the tooltip on a more specific widget.

Test Plan:
`ninja && src/qt/bitcoin-qt -regtest`
Check the tooltip in the amount field on the Send tab.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8189

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