avocado.core.output: Add missing docstrings

So we get even more complete API documentation.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.utils.process: Improving docstrings

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #40 from avocado-framework/docstring-improvements

Docstring improvements