Document repairing file cache holes

[PVince81]

• update docs:

Fixing individual user (no downtime required):

1. run occ files:scan $userId --repair. Replace $userId with the id of the affected user

Fixing all users (downtime required):

1. backup the database!
2. switch the instance to single user mode: occ maintenance:singleuser --on. Note that no files can be accessed by anyone in this mode. (maintenance mode doesn't work as it disables the occ files:scan command currently, see Allow files:scan in maintenance mode owncloud/core#28561)
3. run occ files:scan --all --repair
4. switch the instance back: occ maintenance:singleuser --off

• add entry to FAQ about ghost folders (doubled) pointing to the documentation

Ref PR with more info: owncloud/core#28253

@tomneedham @kdslkdsaldsal

[mmattel]

Needs also a (brief) description of the issue where a repair step helps to fix.
Else this is just run by "lets repair and see if something was found.."

[PVince81]

Some possible symptoms:

• folders having the same entry twice in the web UI (ghost folder) which can also lead to strange error messages in the desktop client
• any situation where entering a visible folder doesn't seem to lead into said folder