Clarify that json.message_key is optional in Filebeat (elastic#3055)

I reordered the options based on importance (I put the optional config setting at the end).

And I changed the wording to further clarify that the `json.message_key` setting is optional.

Fixes elastic#2864

filebeat/docs/reference/configuration/filebeat-options.asciidoc

@@ -320,24 +320,26 @@ Example configuration:
 
 [source,yaml]
 -------------------------------------------------------------------------------------
-json.message_key: log
 json.keys_under_root: true
 json.add_error_key: true
+json.message_key: log
 -------------------------------------------------------------------------------------
 
-
-*`message_key`*:: JSON key on which to apply the line filtering and multiline settings. This key must be top level
-and its value must be string, otherwise it is ignored. If no text key is defined, the line
-filtering and multiline features cannot be used.
-
 *`keys_under_root`*:: By default, the decoded JSON is placed under a "json" key in the output document.
 If you enable this setting, the keys are copied top level in the output document. The default is false.
 
 *`overwrite_keys`*:: If `keys_under_root` and this setting are enabled, then the values from the decoded
 JSON object overwrite the fields that Filebeat normally adds (type, source, offset, etc.) in case of conflicts.
 
 *`add_error_key`*:: If this setting is enabled, Filebeat adds a "json_error" key in case of JSON
-unmarshaling errors or when a text key is defined in the configuration but cannot be used.
+unmarshalling errors or when a `message_key` is defined in the configuration but cannot be used.
+
+*`message_key`*:: An optional configuration setting that specifies a JSON key on
+which to apply the line filtering and multiline settings. If specified the
+key must be at the top level in the JSON object and the value associated with
+the key must be a string, otherwise no filtering or multiline aggregation will
+occur.
+
 
 [[multiline]]
 ===== multiline