Update export_import.md

Grafana v5.3.4 shows a new checkbox in the export modal "Export for sharing externally". If the checkbox is not checked then the "__inputs" section wont be included into the exported JSON file, would be great to add that note into the documentation for others to avoid confusions.

docs/sources/reference/export_import.md

@@ -107,3 +107,5 @@ it as usual and then update the data source option in the metrics tab so that th
 data source. Another alternative is to open the json file in a a text editor and update the data source properties
 data source. Another alternative is to open the json file in a a text editor and update the data source properties
 to value that matches a name of your data source.
 to value that matches a name of your data source.
 
 
+## Note
+In Grafana v5.3.4+ the export modal has new checkbox for sharing for external use (other instances). If the checkbox is not checked then the `__inputs` section will not be included in the exported JSON file.