qapi/common.py: Convert comments into docstrings, and elaborate

qapi/common.py: Convert comments into docstrings, and elaborate

As docstrings, they'll show up in documentation and IDE help.

The docstring style being targeted is the Sphinx documentation
style. Sphinx uses an extension of ReST with "domains". We use the
(implicit) Python domain, which supports a number of custom "info
fields". Those info fields are documented here:
https://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html#info-field-lists

Primarily, we use `:param X: descr`, `:return[s]: descr`, and `:raise[s]
Z: when`. Everything else is the Sphinx dialect of ReST.

(No, nothing checks or enforces this style that I am aware of. Sphinx
either chokes or succeeds, but does not enforce a standard of what is
otherwise inside the docstring. Pycharm does highlight when your param
fields are not aligned with the actual fields present. It does not
highlight missing return or exception statements. There is no existing
style guide I am aware of that covers a standard for a minimally
acceptable docstring. I am debating writing one.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20201009161558.107041-17-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>