Document borgmatic's limitations around parallelism—both its own and Borg's (#957).

NEWS

@@ -1,4 +1,7 @@
 2.0.11.dev0
+ * #957: Document borgmatic's limitations around parallelism—both its own and Borg's. See the
+   documentation for more information:
+   https://torsion.org/borgmatic/how-to/make-per-application-backups/#limitations
  * #1165: Fix for when the systemd service directories (RuntimeDirectory and StateDirectory) each
    contain multiple paths.
  * #1168: Fix for the "list", "info", and "delete" options in "extra_borg_options" being ignored

docs/how-to/make-per-application-backups.md

@@ -59,6 +59,20 @@ entry using borgmatic's `--config` flag instead of relying on
 `/etc/borgmatic.d`.
 
 
+## Limitations
+
+borgmatic does not currently support its own parallelism—being run multiple
+times on the same machine simultaneously. In particular, many of the [data
+source
+hooks](https://torsion.org/borgmatic/reference/configuration/data-sources/) rely
+on global borgmatic runtime files which can't be shared across processes, and
+therefore multiple borgmatic instances on the same machine would interfere with
+each other.
+
+A single borgmatic instance also doesn't currently support running multiple Borg
+instances in parallel on the same machine.
+
+
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