After reading the text and shuffling the order a bit, I think this
has an easier to follow structure. A few highlights:

* An Avocado test is really a method, that is, for many methods we
  have many tests.
* Suppress the attempt to briefly explain the Multiplexer here,
  giving instead a link to a more complete documentation.

Changes from v1:
* Mention the function naming conventions for the test methods and
  link to PEP 8 relevant section for functions and classes.
* Break into finer grained sections.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

[fixup] docs: simplify Writing Avocado Test section text
Signed-off-by: NCleber Rosa <crosa@redhat.com>