Renamed the method to with_resource because I felt enter_context was confusing as a method on Context. The new name reflects that the resource would be managed with the with statement. Moved the docs to advanced.rst and rewrote them, along with the doc strings for with_resource and call_on_close. ExitStack.__enter__ doesn't have to be called, and doesn't return a different object, so now the implementation only tracks a single attribute called _exit_stack, and calls the close method instead of __exit__.