Clarify input type configuration options (elastic#19284) (elastic#21502)

Clarify that all input type configuration options must be specified within the external configuration file: elastic#19148

Co-authored-by: Pius <pius@elastic.co>

filebeat/docs/reload-configuration.asciidoc

@@ -33,9 +33,11 @@ definitions.
 
 TIP: The first line of each external configuration file must be an input
 definition that starts with `- type`. Make sure you omit the line
-+{beatname_lc}.config.inputs+ from this file.
-
-For example:
++{beatname_lc}.config.inputs+ from this file. All <<filebeat-input-types,`input type configuration options`>> 
+must be specified within each external configuration file.  Specifying these
+configuration options at the global `filebeat.config.inputs` level is not supported.
+
+Example external configuration file:
 
 [source,yaml]
 ------------------------------------------------------------------------------