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>
avocado.result: Docstring fixes and improvements

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.sysinfo: Improve docstrings

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

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

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Fix install instructions in GetStartedGuide

The commands described did not work under some circumstances,
so change them to safer versions.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>