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>

completions/bash/terractl

@@ -141,7 +141,7 @@ _terractl() {
         cword=$((cword - 2))
         commands+=" --help --version"
     else
-        commands+=" copy move remote daemon prepare run --host --help --version"
+        commands+=" copy move remote daemon prepare run journal-access --host --help --version"
     fi
 
     if (( cword == 1 )); then
@@ -151,7 +151,7 @@ _terractl() {
 
     local command=${words[1]}
     case $command in
-        list|help|-h|--help|version|-V|--version)
+        list|journal-access|help|-h|--help|version|-V|--version)
             return ;;
         new)
             # new <new name> [vanilla|tmodloader] [port]: the name is new, so nothing to offer