docs: Add parallel mode

docs/chapters/targeting.rst

@@ -35,6 +35,22 @@ This value can be changed using ``bastille config TARGET set priority VALUE``.
 
 This value will be shown using ``bastille list all``.
 
+Parallel Mode
+-------------
+
+Any command that supports multiple targets, also supports parallel mode. This means that
+Bastille will run the command on multiple jails at a single time, depending on the value
+given.
+
+
+To use parallel mode, run ``bastille -p 4 pkg ALL update``, for exapmle, to start
+updating pacakges in all jails, 4 processes at a time.
+
+Note that the ``-p`` option should follow the main ``bastille`` command, and not the sub-command.
+
+To run an unlimited number of processes, simply set this value to 0 when running
+a command.
+
 Examples: Containers
 --------------------