docs: auto generate backend options documentation

This inserts the output of "rclone help backend xxx" into the help
pages for each backend.

docs/content/opendrive.md

@@ -93,8 +93,30 @@ OpenDrive allows modification times to be set on objects accurate to 1
 second. These will be used to detect whether objects need syncing or
 not.
 
-<!--- autogenerated options start - edit in backend/backend.go options  -->
-<!--- autogenerated options stop  -->
+<!--- autogenerated options start - DO NOT EDIT, instead edit fs.RegInfo in backend/opendrive/opendrive.go then run make backenddocs -->
+### Standard Options
+
+Here are the standard options specific to opendrive (OpenDrive).
+
+#### --opendrive-username
+
+Username
+
+- Config:      username
+- Env Var:     RCLONE_OPENDRIVE_USERNAME
+- Type:        string
+- Default:     ""
+
+#### --opendrive-password
+
+Password.
+
+- Config:      password
+- Env Var:     RCLONE_OPENDRIVE_PASSWORD
+- Type:        string
+- Default:     ""
+
+<!--- autogenerated options stop -->
 
 ### Limitations ###