rc: allow fs= params to be a JSON blob

docs/content/rc.md

@@ -378,6 +378,55 @@ call and taken by the [options/set](#options-set) calls as well as the
 - `BandwidthSpec` - this will be set and returned as a string, eg
   "1M".
 
+## Specifying remotes to work on
+
+Remotes are specified with the `fs=`, `srcFs=`, `dstFs=`
+parameters depending on the command being used.
+
+The parameters can be a string as per the rest of rclone, eg
+`s3:bucket/path` or `:sftp:/my/dir`. They can also be specified as
+JSON blobs.
+
+If specifyng a JSON blob it should be a object mapping strings to
+strings. These values will be used to configure the remote. There are
+3 special values which may be set:
+
+- `type` -  set to `type` to specify a remote called `:type:`
+- `_name` - set to `name` to specify a remote called `name:`
+- `_root` - sets the root of the remote - may be empty
+
+One of `_name` or `type` should normally be set. If the `local`
+backend is desired then `type` should be set to `local`. If `_root`
+isn't specified then it defaults to the root of the remote.
+
+For example this JSON is equivalent to `remote:/tmp`
+
+```
+{
+    "_name": "remote",
+    "_path": "/tmp"
+}
+```
+
+And this is equivalent to `:sftp,host='example.com':/tmp`
+
+```
+{
+    "type": "sftp",
+    "host": "example.com",
+    "_path": "/tmp"
+}
+```
+
+And this is equivalent to `/tmp/dir`
+
+```
+{
+    type = "local",
+    _ path = "/tmp/dir"
+}
+```
+
 ## Supported commands
 {{< rem autogenerated start "- run make rcdocs - don't edit here" >}}
 ### backend/command: Runs a backend command. {#backend-command}