Update documentation to async code (#331) (#332)

* Update documentation to async code (#331)

This commit updates the documentation to the latest changes added
with pycapnp 2.0.0.

* Remove non existing classes/functions from the reference documentation
* Adapt the quickstart to the latest changes. Mainly to new rpc handling,
  that now exlusively is done through asyncio.

* DOC: Add section about send and receive messages over a socket

Since #313 it is possible to read and write messages over a socket.
This commit adds a small section for read and write in the quickstart.

docs/conf.py

@@ -307,6 +307,6 @@ epub_copyright = "2013, Author"
 # Allow duplicate toc entries.
 # epub_tocdup = True
 
-intersphinx_mapping = {"http://docs.python.org/": None}
+intersphinx_mapping = {"<name>": ("http://docs.python.org/", None)}
 
 smv_branch_whitelist = r"^master$"