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