doc: pytest: Document the test_net_boot test

doc: pytest: Document the test_net_boot test

Add the test_net_boot.py test to the generated documentation. While most
of this was already commented correctly for inclusion the biggest
problem was examples of code without a code-block notation. This in turn
broke parsing. Add the missing notations. We also must have the comment
prior to any import lines or it will not be seen as a comment on the
overall file and thus not included.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>