Clarifying documentation on filename placeholders

The documentation text now better describes what the code does. It is now clear that the time is not included, and that all date placeholders are either zero-padded or ISO-formatted.

https://github.com/jonaswinkler/paperless-ng/blob/05c36f91cfa5613bdea5b3c490774cc40b586eeb/src/documents/file_handling.py#L164-L171

docs/advanced_usage.rst

@@ -221,14 +221,14 @@ Paperless provides the following placeholders withing filenames:
 * ``{document_type}``: The name of the document type, or "none".
 * ``{tag_list}``: A comma separated list of all tags assigned to the document.
 * ``{title}``: The title of the document.
-* ``{created}``: The full date and time the document was created.
+* ``{created}``: The full date (ISO format) the document was created.
 * ``{created_year}``: Year created only.
-* ``{created_month}``: Month created only (number 1-12).
-* ``{created_day}``: Day created only (number 1-31).
-* ``{added}``: The full date and time the document was added to paperless.
+* ``{created_month}``: Month created only (number 01-12).
+* ``{created_day}``: Day created only (number 01-31).
+* ``{added}``: The full date (ISO format) the document was added to paperless.
 * ``{added_year}``: Year added only.
-* ``{added_month}``: Month added only (number 1-12).
-* ``{added_day}``: Day added only (number 1-31).
+* ``{added_month}``: Month added only (number 01-12).
+* ``{added_day}``: Day added only (number 01-31).
 
 
 Paperless will try to conserve the information from your database as much as possible.