Developer documentation update involving module invocation (ansible#5…

…5747)

* Update docs for the 2.7 change to AnsiballZ which invokes modules with one
  less Python interpreter

* Add a section on how module results are returned and on trust between modules, action plugins, and the executor.

* Update docs/docsite/rst/dev_guide/developing_program_flow_modules.rst

Co-Authored-By: abadger <a.badger@gmail.com>