drive: warn when non-exportable Google documents are skipped - #9475

Non-exportable Google documents such as Google My Maps, Sites and
Forms have no export format, so rclone silently leaves them out of all
listings. During a server side move/copy this means they are quietly
left behind.

Emit a one-off notice when such a document is skipped, telling the
user the files are invisible to rclone and pointing them at
--drive-show-all-gdocs to include them in server side copies and
moves.

backend/drive/drive.go

@@ -144,6 +144,7 @@ var (
 	_importFormats   map[string][]string           // allowed import MIME type conversions
 	templatesOnce    sync.Once                     // parse link templates only once
 	_linkTemplates   map[string]*template.Template // available link types
+	gdocsWarnOnce    sync.Once                     // warn once about skipped non-exportable Google documents
 )
 
 // rwChoices type for fs.Bits
@@ -1686,6 +1687,9 @@ func (f *Fs) newObjectWithExportInfo(
 		// If item MimeType is in the ExportFormats then it is a google doc
 		if !isDocument {
 			fs.Debugf(remote, "Ignoring unknown document type %q", info.MimeType)
+			gdocsWarnOnce.Do(func() {
+				fs.Logf(remote, "Skipping unexportable google documents. Use --drive-show-all-gdocs to include them in server side copy and move", info.MimeType)
+			})
 			return nil, fs.ErrorObjectNotFound
 		}
 		if extension == "" {