Document ".okular" filename extension

Summary:
In general Okular makes it very hard to get the filename extension for
document archives wrong when saving. Still it would be nice to document
this.

BUG: 291176

Test Plan: `meinproc5` still happy.

Reviewers: lueck, yurchor

Reviewed By: yurchor

Subscribers: #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D9577