DOC: Plugins doc should reflect new menu layout

[pllim]

#619 changed "operations" menu layout but plugin instructions are not updated accordingly, as reported by @mkb0517 in https://github.com/ejeschke/ginga/issues/603#issuecomment-405052500 . A sweep through all the plugins to correct this is necessary.

[pllim]

Following discussions from #671

@ejeschke -- "I wonder about the whole idea of 'hardcoding' the menu locations of plugins in the documentation, given that the user can actually create their own menu order quite easily! Maybe we need some language that allows for this flexibility."

@mkb0517 -- "Maybe if we add “by default” to it? I’d assume that by the time they were making they were making their own menus, they’d be familiar enough with the layout that they would disregard that part."