cfg: adds info about local img uploader to docs

conf/sample.ini

@@ -417,7 +417,7 @@ log_queries =
 #################################### External image storage ##########################
 [external_image_storage]
 # Used for uploading images to public servers so they can be included in slack/email messages.
-# you can choose between (s3, webdav, gcs, azure_blob)
+# you can choose between (s3, webdav, gcs, azure_blob, local)
 ;provider =
 
 [external_image_storage.s3]
@@ -442,3 +442,6 @@ log_queries =
 ;account_name =
 ;account_key =
 ;container_name =
+
+[external_image_storage.local]
+# does not require any configuration

docs/sources/installation/configuration.md

@@ -766,7 +766,7 @@ Time to live for snapshots.
 These options control how images should be made public so they can be shared on services like slack.
 
 ### provider
-You can choose between (s3, webdav, gcs, azure_blob). If left empty Grafana will ignore the upload action.
+You can choose between (s3, webdav, gcs, azure_blob, local). If left empty Grafana will ignore the upload action.
 
 ## [external_image_storage.s3]