Clarifying documentation on filename placeholders

[denilsonsa]

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.

paperless-ng/src/documents/file_handling.py

Lines 164 to 171 in 05c36f9

	created=datetime.date.isoformat(doc.created),
	created_year=doc.created.year if doc.created else "none",
	created_month=f"{doc.created.month:02}" if doc.created else "none", # NOQA: E501
	created_day=f"{doc.created.day:02}" if doc.created else "none",
	added=datetime.date.isoformat(doc.added),
	added_year=doc.added.year if doc.added else "none",
	added_month=f"{doc.added.month:02}" if doc.added else "none",
	added_day=f"{doc.added.day:02}" if doc.added else "none",