qt: add missing tooltips to options menu settings

Summary: This is a backport of core-gui#419

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

check the tooltips by hovering the mouse on the "Size of database cache" and "number of script verification threads" labels in the Options menu

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18576