esx_vshpere_datastore_io: Add information to the manpage

Change-Id: Ie1f7d0cc0e28b8be2af9014e9ce6627954eb009b

Author: racicLuka

cmk/plugins/vsphere/checkman/esx_vsphere_datastore_io

@@ -5,6 +5,10 @@ license: GPLv2
 distribution: check_mk
 description:
  This check measures the number of written and read bytes and the latency for each datastore on an ESX host system.
+ It is important to note that ESX host systems do not always report valid data for the metrics mentioned above.
+ Metrics with currently invalid data are omitted until the ESX host system starts to report valid data.
+ If there is no valid data for any metric, the service will go stale.
+
  You can configure levels for the read, write and latency values.
 
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