Enhance documentation regarding internal icinga config sync check (#10101)

* Update 10-icinga-template-library.md

Explicitly name the config-sync check feature of the icinga check, as before this was a little bit too undocumented making it unknown to me.
Also mention where the check has to executed in order to bring the desired results.

* Update 15-troubleshooting.md

Add 4h typical error point for configuration stored outside of /etc/icinga2/zones.d. For when a non-distributed setup was migrated to a distributed setup.
Also link to the internal icinga CheckCommand to promote its existance.

* Update 15-troubleshooting.md

Remove "-" from link

* Revert "Update 15-troubleshooting.md"

This reverts commit bb25ba3ff5.

* Update AUTHORS

Add myself to AUTHORS

* Update doc/15-troubleshooting.md

Co-authored-by: alvar <8402811+oxzi@users.noreply.github.com>

* Update doc/10-icinga-template-library.md

Co-authored-by: alvar <8402811+oxzi@users.noreply.github.com>

* Update doc/15-troubleshooting.md

Co-authored-by: alvar <8402811+oxzi@users.noreply.github.com>

---------

Co-authored-by: alvar <8402811+oxzi@users.noreply.github.com>

AUTHORS

@@ -52,6 +52,7 @@ Christian Gut <cycloon@is-root.org>
 Christian Harke <ch.harke@gmail.com>
 Christian Jonak <christian@jonak.org>
 Christian Lehmann <christian_lehmann@gmx.de>
+Christian Lauf <github.com@christian-lauf.info>
 Christian Loos <cloos@netsandbox.de>
 Christian Schmidt <github@chsc.dk>
 Christopher Peterson <3893680+cspeterson@users.noreply.github.com>

doc/10-icinga-template-library.md

@@ -75,8 +75,10 @@ plugin scripts.
 
 ### icinga <a id="itl-icinga"></a>
 
-Check command for the built-in `icinga` check. This check returns performance
+Check command for the built-in `icinga` check. This check returns performance data for the current Icinga instance,
-data for the current Icinga instance, reports as warning if the last reload failed and optionally allows for minimum version checks.
+reports as warning if the last reload or config sync failed and optionally allows for minimum version checks.
+
+For the config sync check to work, it must be run on the satellite or agent.
 
 Custom variables passed as [command parameters](03-monitoring-basics.md#command-passing-parameters):
 

doc/15-troubleshooting.md

@@ -1698,6 +1698,9 @@ Typical errors are:
 * The api feature doesn't [accept config](06-distributed-monitoring.md#distributed-monitoring-top-down-config-sync). This is logged into `/var/lib/icinga2/icinga2.log`.
 * The received configuration zone is not configured in [zones.conf](04-configuration.md#zones-conf) and Icinga denies it. This is logged into `/var/lib/icinga2/icinga2.log`.
 * The satellite/agent has local configuration in `/etc/icinga2/zones.d` and thinks it is authoritive for this zone. It then denies the received update. Purge the content from `/etc/icinga2/zones.d`, `/var/lib/icinga2/api/zones/*` and restart Icinga to fix this.
+* Configuration parts stored outside of `/etc/icinga2/zones.d` on the master, for example a constant in `/etc/icinga2/constants.conf`, are then missing on the satellite/agent.
+
+Note that if set up, the [built-in icinga CheckCommand](10-icinga-template-library.md#icinga) will notify you in case the config sync wasn't successful.
 
 #### New configuration does not trigger a reload <a id="troubleshooting-cluster-config-sync-no-reload"></a>