i18n: Change style of sentence

1. Changing the style of the sentence.
2. Unifying the sentence with Talk Nextcloud.

Signed-off-by: Valdnet <47037905+Valdnet@users.noreply.github.com>

app/src/main/res/values/strings.xml

@@ -576,7 +576,7 @@
     <string name="nc_expire_message_one_day">1 day</string>
     <string name="nc_expire_message_eight_hours">8 hours</string>
     <string name="nc_expire_message_one_hour">1 hour</string>
-    <string name="nc_expire_messages_explanation">Expire chat messages after a certain time. Files shared into the chat will only be unshared from the conversation but are not deleted for the owner.</string>
+    <string name="nc_expire_messages_explanation">Chat messages expire after a while. Note: Files shared in chat will not be deleted for the owner, but will no longer be shared in the chat.</string>
 
 
 </resources>