shell: move documentation out of kernel

The shell is not a kernel feature, it is more of a subsystem.

Change-Id: Iaba60b2086ddfe77af427d70b8fc8d06a8bebe14
Signed-off-by: Anas Nashif <anas.nashif@intel.com>

doc/kernel/common/common.rst

@@ -13,4 +13,3 @@ microkernel applications and nanokernel applications.
    common_kernel_clocks.rst
    common_atomic.rst
    common_float.rst
-   common_shell.rst

doc/subsystems/subsystems.rst

@@ -14,3 +14,4 @@ to applications.
    usb/usb.rst
    sensor
    power_management.rst
+   shell

drivers/console/console_handler_shell.c

@@ -240,6 +240,7 @@ static int show_help(int argc, char *argv[])
 		for (module = 0; module < NUM_OF_SHELL_ENTITIES; module++) {
 			printk("%s\n", __shell_cmd_start[module].module_name);
 		}
+		printk("\nTo select a module, enter 'set_module <module name>'.\n");
 	}
 
 	return 0;