Address SSH batch mode in docs (fixes #2202) (#2270)

docs/usage.rst

@@ -729,3 +729,11 @@ for e.g. regular pruning.
 Further protections can be implemented, but are outside of Borg's scope. For example,
 file system snapshots or wrapping ``borg serve`` to set special permissions or ACLs on
 new data files.
+
+SSH batch mode
+~~~~~~~~~~~~~~
+
+When running |project_name| using an automated script, ``ssh`` might still ask for a password,
+even if there is an SSH key for the target server. Use this to make scripts more robust::
+
+    export BORG_RSH='ssh -oBatchMode=yes'