added notes regarding the alias flush function; Fixes #4196 (#298)

source/manual/aliases.rst

@@ -72,6 +72,15 @@ As you can see there are multiple IP addresses for this domain.
 Hosts type Aliases can contain exclusion hosts.
 Exclusion addresses starts with "!" sign (eg !192.168.0.1) and can be used to exclude hosts from Network Group Aliases.
 
+.. Warning::
+
+    Please note thet the **Flush** action is not persistent!
+
+    **"flush"** means flush the current contents of the alias, which will be repopulated when it's not an external type,
+    so flush in most cases isn't very useful.
+
+    Same behaviour applies to the API call `alias_util flush </development/api/core/firewall.html>`_
+
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 Networks
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