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

doc/index.docbook

@@ -457,7 +457,7 @@ Context menu actions like Rename Bookmarks etc.)
 			<para>Using the context menu either in the <guilabel>Reviews</guilabel> view of the navigation panel or in the main window you can open a <guilabel>Pop up Note</guilabel> for any kind of annotation and add or edit comments.</para>
 			<para>Annotations are not only limited to &PDF; files, they can be used for any format &okular; supports.</para>
 			<para>
-                &okular; has the "document archiving" feature. This is an &okular;-specific format for carrying the document plus various metadata related to it (currently only annotations). You can save a "document archive" from the open document by choosing <menuchoice><guimenu>File</guimenu><guisubmenu>Save As</guisubmenu></menuchoice> and selecting <guilabel>Okular document archive</guilabel> in the <guilabel>Filter</guilabel> selector. To open an &okular; document archive, just open it with &okular; as it would be &eg; a &PDF; document.
+                &okular; has the "document archiving" feature. This is an &okular;-specific format for carrying the document plus various metadata related to it (currently only annotations). You can save a "document archive" from the open document by choosing <menuchoice><guimenu>File</guimenu><guisubmenu>Save As</guisubmenu></menuchoice> and selecting <guilabel>Okular document archive</guilabel> in the <guilabel>Filter</guilabel> selector. Documents saved this way will get <filename>.okular</filename> as their filename extension. To open an &okular; document archive, just open it with &okular; as it would be &eg; a &PDF; document.
 			</para>
 			<para>
 			  You can <emphasis>also</emphasis> save annotations directly into &PDF; files. You can use <menuchoice><guimenu>File</guimenu> <guimenuitem>Save</guimenuitem></menuchoice> to save it over the current file or <menuchoice><guimenu>File</guimenu> <guimenuitem>Save As...</guimenuitem></menuchoice> to save it to a new file.