Document AbstractClient::minSize() and AbstractClient::maxSize()

Summary:
Clarify that AbstractClient::minSize() and AbstractClient::maxSize()
specify the minimum and the maximum client size.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26829