FAQ/doc updates: Provide info that deleted files are not handled on extract/restore

[eharris]

This is primarily a request for FAQ and documentation updates.

The FAQ should make it clear (probably under both the Which file types, attributes, etc. are not preserved? section, and Are there other known limitations?) that deleted files (files which did not exist in the backup being restored) are not removed if they exist in the extract destination. So a restore/extract is not currently able to perform an exact "point-in-time" restore, UNLESS the destination location did not previously exist, which also requires enough spare disk space to hold an additional copy of the entire backup, and takes forever for large backups. (See #963, #1828, #1986, #2872)

A workaround for this missing functionality, which should probably be added as an additional FAQ item is:

1. Use borg create to create a new (temporary) archive of the current on-disk state
2. Use borg diff to compare the past archive with the one created in step 1
3. Parse the output from step 2 and delete any files that are listed as removed
4. Use borg extract to restore the rest of the backup
5. Use borg delete to remove the temporary archive created in step 1

A good comment about why this functionality isn't present yet is here: #1986 (comment)

[eharris]

This also helps explain (to unfamiliar users) why there is no borg restore verb. It really is just extracting the files contained in the archive, not restoring a prior state.

[ThomasWaldmann]

To make it even more clear:

• it is restoring to a prior state if you start from an empty directory.
• it is not transforming a directory with some arbitrary content to a prior state though.