warn when the services can't reach moved folders

- list, new, import and enable check TERRARIA_BASE and TERRARIA_DIST
  against the units' ProtectSystem/ProtectHome/ReadWritePaths
  (terraria@ always, terractld when it's on) and print the drop-in
  to add. prepare explains why it can't create an instance folder.
- Guide: bind mounts and drop-ins under "Changing locations", and a
  new section for instances or zips under /home.
- journal-access in tab completion and the command reference.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

config/default/terractl

@@ -1,13 +1,16 @@
 # Overrides for terractl. Uncomment to change.
 # Changes apply to new commands and to each instance's next start.
 
-# Where instances live. If you change this, the service must also be allowed
-# to write there:  sudo systemctl edit terraria@.service
+# Where instances live. To use another disk, it's simplest to mount it at the
+# default location instead. If you change this, the service must also be
+# allowed to write there:  sudo systemctl edit terraria@.service
 #   [Service]
 #   ReadWritePaths=/your/new/path
+# Under /home it needs more than that. 'terractl list' warns about what's
+# missing; see "Changing locations" in the guide.
 #TERRARIA_BASE=/var/lib/terraria/instances
 
-# Where server zips are read from
+# Where server zips are read from. Outside /home, no drop-in is needed.
 #TERRARIA_DIST=/var/lib/terraria/dist
 
 # Seconds to wait for a server to save and exit on stop (keep below 90)