shell: doc: adding information on command execution

Supplemental information on shell command execution. Added an
illustration and four examples showing how the shell will call
the handler and what it will pass as an argument.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>

Author: Jakub Rzeszutko

doc/services/shell/images/execution.png

@@ -285,6 +285,44 @@ and a function :c:func:`shell_execute_cmd`, as shown in this example:
 Enable the DUMMY backend by setting the Kconfig
 :kconfig:option:`CONFIG_SHELL_BACKEND_DUMMY` option.
 
+Commands execution example
+--------------------------
+
+Let's assume a command structure as in the following figure, where:
+
+* :c:macro:`root_cmd` - root command without a handler
+* :c:macro:`cmd_xxx_h` - command has a handler
+* :c:macro:`cmd_xxx` - command does not have a handler
+
+.. image:: images/static_cmd.PNG
+      :align: center
+      :alt: Command tree with static commands.
+
+Example 1
+^^^^^^^^^
+Sequence: :c:macro:`root_cmd` :c:macro:`cmd_1_h` :c:macro:`cmd_12_h`
+:c:macro:`cmd_121_h` :c:macro:`parameter` will execute command
+:c:macro:`cmd_121_h` and :c:macro:`parameter` will be passed as an argument.
+
+Example 2
+^^^^^^^^^
+Sequence: :c:macro:`root_cmd` :c:macro:`cmd_2` :c:macro:`cmd_22_h`
+:c:macro:`parameter1` :c:macro:`parameter2` will execute command
+:c:macro:`cmd_22_h` and :c:macro:`parameter1` :c:macro:`parameter2`
+will be passed as an arguments.
+
+Example 3
+^^^^^^^^^
+Sequence: :c:macro:`root_cmd` :c:macro:`cmd_1_h` :c:macro:`parameter1`
+:c:macro:`cmd_121_h` :c:macro:`parameter2` will execute command
+:c:macro:`cmd_1_h` and :c:macro:`parameter1`, :c:macro:`cmd_121_h` and
+:c:macro:`parameter2` will be passed as an arguments.
+
+Example 4
+^^^^^^^^^
+Sequence: :c:macro:`root_cmd` :c:macro:`parameter` :c:macro:`cmd_121_h`
+:c:macro:`parameter2` will not execute any command.
+
 
 Command handler
 ----------------