docs: auto generate backend options documentation

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

docs/content/http.md

@@ -126,5 +126,21 @@ without a config file:
 
     rclone lsd --http-url https://beta.rclone.org :http:
 
-<!--- autogenerated options start - edit in backend/backend.go options  -->
-<!--- autogenerated options stop  -->
+<!--- autogenerated options start - DO NOT EDIT, instead edit fs.RegInfo in backend/http/http.go then run make backenddocs -->
+### Standard Options
+
+Here are the standard options specific to http (http Connection).
+
+#### --http-url
+
+URL of http host to connect to
+
+- Config:      url
+- Env Var:     RCLONE_HTTP_URL
+- Type:        string
+- Default:     ""
+- Examples:
+    - "https://example.com"
+        - Connect to example.com
+
+<!--- autogenerated options stop -->