Improve readability of a tooltip

Summary:
The tooltip for {nav Capture the current pop-up only} was very wide,
making it hard to read. Changing the wording and adding a line break
improves the situation.

Test Plan:
Hover mouse over the checkbox, notice reading the tooltip on two lines
is easier.

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Differential Revision: https://phabricator.kde.org/D10372