Update filename format docs to discuss uniqueness

[melithine]

The docs don't currently talk about not using mediaid can cause files to get overwritten or skipped as already downloaded. We should update that page to provide better guidance to users and warn what could happen if they are using a format not unique enough. Discussed in Discord with koopio122 around https://discord.com/channels/1198332760947966094/1198337942641451099/1272272088052400302