KSqueezedTextLabel: Small improvements to API docs

Summary:
Expand description, add missing documentation.

"Above" in source code does not imply "above" in generated API docs,
better use a reference.

Test Plan: Output of kapidox looks better.

Reviewers: Frameworks, dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7161