Add documentation about @rclick method for sub-menus

[boltex]

I am going to assign this to 6.7.6 (for now) so that it doesn't interfere with leoJS.

Specifically, that the intermediate node has to be empty for sub menus to exist as children of rclick node, and so on. :)

as per https://groups.google.com/g/leo-editor/c/rNTMxpAuFb8/m/rOxkvyGTAQAJ