Add tooltip for quick search

Summary:
Add a tooltip to the quick search line edit in the process view to inform the user that it is possible to use comma separated search terms and regular expressions.

We can also reformulate the text if you have better ideas.

BUG: 403923

Reviewers: Plasma, cfeck, broulik, mart

Reviewed By: Plasma, mart

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19312