Add some documentation on how to enhance PDF printing in Okular
Summary: It is sometimes untrivial for ordinary user to find the right options to print PDF. It should be documented somehow.
Test Plan: Tested woth checkXML5
Reviewers: Okular, Documentation, ngraham
Reviewed By: ngraham
Subscribers: ngraham, okular-devel, kde-doc-english
Tags: Okular, Documentation
Differential Revision: https://phabricator.kde.org/D17749