Documentation: Fix usermod on Debian.

Fixes #5653

doc/2.4-setting-up-livestatus.md

@@ -30,6 +30,11 @@ In order for queries and commands to work you will need to add your query user
 
     # usermod -a -G icingacmd www-data
 
+> **Note**
+>
+> Debian packages use `nagios` as default user/group. Therefore change `icingacmd` to
+> `nagios`.
+
 > **Note**
 >
 > Change "www-data" to the user you're using to run queries.

doc/3.08-external-commands.md

@@ -31,6 +31,11 @@ a forced service check:
 
     # usermod -G -a icingacmd www-data
 
+> **Note**
+>
+> Debian packages use `nagios` as default user/group. Therefore change `icingacmd` to
+> `nagios`.
+
 ### <a id="external-command-list"></a> External Command List
 
 A list of currently supported external commands can be found on the