[UX] Provide some help text about temporary files in the "Manage files" form

This came up in a Zulip thread: https://backdrop.zulipchat.com/#narrow/stream/218635-Backdrop/topic/Dele...

@richarda78

Neither removing an image from a node or deleting a node removes images from Backdrop. Is there something I'm missing, like a list of redundant images?

@albanycomputers

You can delete images manually through "Content--> manage files ... Try running Cron.

@indigoxela

Deleting unused images (not in use in any node/field/...) happens on cron runs - after a delay.

You can see "candidates" on the files overview on admin/content/files. "Permanent" images won't get deleted, only "Temporary". And only after a grace time (6 hrs by default).

@richarda78

... yeah I'm pretty sure I looked at manage files, some help text would be great.

I thought that perhaps adding some help text above the file list might help. Here's an initial suggestion for discussion and word smithing:

When the use count of a file drops to 0 (not used anywhere on the site), its status automatically changes from "Permanent" to "Temporary". Files flagged as temporary are then deleted approximately within 6 hours if they remain unused.

Screenshot of what it would look like (added here as a Views header):