Move source actors into their own section; add docs on pretty printing

So I realized that listing sources and getting their contents is all in the "inspecting paused threads" section, which is incorrect, since you can do all that whether you are paused or not.

* I moved the listing sources to "interacting with thread like actors" since it doesn't require a pause.

* I created a section for source actors.

* Moved getting source contents and black boxing stuff to the new source actor section.

* Added docs on pretty printing to the source actor section.