Add further plugin documentation

[dmcgowan]

Add information related to the use of plugins in the scope specification and updated relevant configuration documentation.

[marcusmartins]

@dmcgowan are you planning to address this for Registry/2.7 or should we move it out? I am wondering if it would helpful to add a label to indentify issues that should be in our backlog.

[dmcgowan]

There is probably a little documentation that should be added. My perspective is also that this is not a feature which is useful enough for me to spend time documenting, but I want to ensure there is clarity to the open source operators on what the flags are that we added. It may be more appropriate for the documentation team to add a "recipe" is they find that to be something registry operators may want.