Document limitation about using database hooks and "one_file_system" (#853).

docs/how-to/backup-your-databases.md

@@ -437,19 +437,27 @@ borgmatic's own configuration file. So include your configuration file in
 backups to avoid getting caught without a way to restore a database.
 3. borgmatic does not currently support backing up or restoring multiple
 databases that share the exact same name on different hosts.
-4. Because database hooks implicitly enable the `read_special` configuration,
-any special files are excluded from backups (named pipes, block devices,
-character devices, and sockets) to prevent hanging. Try a command like `find
-/your/source/path -type b -or -type c -or -type p -or -type s` to find such
-files. Common directories to exclude are `/dev` and `/run`, but that may not
-be exhaustive. <span class="minilink minilink-addedin">New in version
-1.7.3</span> When database hooks are enabled, borgmatic automatically excludes
-special files (and symlinks to special files) that may cause Borg to hang, so
-generally you no longer need to manually exclude them. There are potential
-edge cases though in which applications on your system create new special files
-*after* borgmatic constructs its exclude list, resulting in Borg hangs. If that
-occurs, you can resort to the manual excludes described above. And to opt out
-of the auto-exclude feature entirely, explicitly set `read_special` to true.
+4. Because database hooks implicitly enable the `read_special` option, any
+special files are excluded from backups (named pipes, block devices,
+character devices, and sockets) to prevent hanging. Try a command like
+`find /your/source/path -type b -or -type c -or -type p -or -type s` to
+find such files. Common directories to exclude are `/dev` and `/run`, but
+that may not be exhaustive. <span class="minilink minilink-addedin">New in
+version 1.7.3</span> When database hooks are enabled, borgmatic
+automatically excludes special files (and symlinks to special files) that
+may cause Borg to hang, so generally you no longer need to manually exclude
+them. There are potential edge cases though in which applications on your
+system create new special files *after* borgmatic constructs its exclude
+list, resulting in Borg hangs. If that occurs, you can resort to the manual
+excludes described above. And to opt out of the auto-exclude feature
+entirely, explicitly set `read_special` to true.
+5. Database hooks also implicitly enable the `one_file_system` option, which
+means Borg won't cross filesystem boundaries when looking for files to backup.
+This is especially important when running borgmatic in a container, as
+container volumes are mounted as separate filesystems. One work-around is to
+explicitly add each mounted volume you'd like to backup to
+`source_directories` instead of relying on Borg to include them implicitly via
+a parent directory.
 
 
 ### Manual restoration