add explenation of the expire function to the apps description

apps/files_trashbin/appinfo/info.xml

@@ -2,7 +2,20 @@
 <info>
 	<id>files_trashbin</id>
 	<name>Deleted files</name>
-	<description>Keep a copy of deleted files so that they can be restored if needed</description>
+	<description>
+	ownCloud keeps a copy of your deleted files in case you need them again. 
+	To make sure that the user doesn't run out of memory the deleted files app
+	manages the size of the deleted files for the user. By default deleted files
+	stay in the trash bin for 180 days. ownCloud checks the age of the files
+	every time a new files gets moved to the deleted files and remove all files
+	older than 180 days. The user can adjust this value in the config.php by
+	setting the "trashbin_retention_obligation" value.
+
+	Beside that the delted files app take care to never use more that 50% of
+	your currently available free space. If your deleted files exceed this limit
+	ownCloud deletes the oldest versions until it meets the memory usage limit
+	again.
+	</description>
 	<licence>AGPL</licence>
 	<author>Bjoern Schiessle</author>
 	<shipped>true</shipped>

apps/files_versions/appinfo/info.xml

@@ -6,7 +6,25 @@
 	<author>Frank Karlitschek</author>
 	<require>4.93</require>
 	<shipped>true</shipped>
-	<description>Versioning of files</description>
+	<description>
+	ownCloud supports simple version control for files.	The versioning app
+	expires old versions automatically to make sure that
+	the user doesn't run out of space. Following pattern is used to delete
+	old versions:
+	For the first 10 seconds ownCloud keeps one version every 2 seconds;
+	For the first hour ownCloud keeps one version every minute;
+	For the first 24 hours ownCloud keeps one version every hour;
+	For the first 30 days ownCloud keeps one version every day;
+	After the first 30 days ownCloud keeps one version every week.
+
+	The versions are adjusted along this pattern every time a new version gets
+	created.
+
+	Beside that the version app takes care to never use more that 50% of the users
+	currently available free space. If the stored versions exceed this limit
+	ownCloud deletes the oldest versions until it meets the memory usage limit
+	again.
+	</description>
 	<types>
 		<filesystem/>
 	</types>