Improve doc about in-place file import do not play

Steps to reproduce

• 1. import some music with --in-place

- (virtualenv) funkwhale@host:~$ python api/manage.py import_files --in-place --username jocelyn /mnt/partage/Son/Musique/some-song.mp3 
Checking imported paths against settings.MUSIC_DIRECTORY_PATH
Import summary:
- 1 files found matching this pattern: /mnt/partage/Son/Musique/Pulp Fiction.mp3
- 0 files already found in database
- 1 new files
Selected options: in place
Are you sure you want to do this?

Type 'yes' to continue, or 'no' to cancel: yes
1/1 Importing /mnt/partage/Son/Musique/Pulp Fiction.mp3...
Successfully imported 1 tracks
For details, please refer to import batch #15
(virtualenv) funkwhale@host:~$ 

• 2. The song appears in library (from web interface)
• 3. Click « play » on it

What happens ?

It skips directly to the next song, and I can see a 404 error in my browser log on the URL of the song (I guess) : https://myserver.example.com/api/v1/trackfiles/161/serve/?jwt=XXXXXXX.

What is expected

Enjoy the song playing, and HTTP 200 in browser console.

Context

• using latest release 0.14.1
• Installed following strictly the Debian guide, served with nginx.
• ~funkwhale/config/.env contains:

...
MUSIC_DIRECTORY_PATH=/mnt/partage/Son/Musique
...

• songs imported without --in-place works flawlessly

I could provide more debug information (browsing log, querying db, using django ./manage.py shell), if told what and where to look for :-)