Update docs for context lib

[reyang]

This should address most of the context related issues discovered by @c24t in #70.

Part of the effort for #22.

[Oberon00]

Does this PR depend on or replace #70?

[Oberon00]

Apart from the two typos that travis/Sphinx complains about (looks like BaseRuntimeContext is missing), LGTM.

[reyang]

Does this PR depend on or replace #70?

Neither, it is trying to make it easier for #70. We don't expect @c24t to fix all the doc issues for context and metrics.

[reyang]

The remaining failures seem like a Sphinx bug:

/home/travis/build/open-telemetry/opentelemetry-python/opentelemetry-api/src/opentelemetry/context/async_context.py:docstring
of opentelemetry.context.async_context.AsyncRuntimeContext.Slot:1: WARNING:
py:class reference target not found: opentelemetry.context.base_context.Slot
/home/travis/build/open-telemetry/opentelemetry-python/opentelemetry-api/src/opentelemetry/context/thread_local_context.py:docstring
of opentelemetry.context.thread_local_context.ThreadLocalRuntimeContext.Slot:1: WARNING:
py:class reference target not found: opentelemetry.context.base_context.Slot

It seems Sphinx got confused while trying to find the base class "class reference target not found: opentelemetry.context.base_context.Slot", the path is wrong.

@c24t any idea how to workaround this? I'll search and see if there are some solution.
BTW, I think we probably don't need to document the actual context implementation, the module level doc should provide sufficient information.

[c24t]

That looks like a sphinx bug to me, you may be able to track down the problem in https://github.com/agronholm/sphinx-autodoc-typehints/blob/master/sphinx_autodoc_typehints.py.

[c24t]

The changes LGTM, but hopefully we can find a way to fix the errors in sphinx before merging this to avoid having a broken build on master. If there's no way around it we can also disable the docs build check for now.

[reyang]

I've temporarily removed the thread_local_context and async_context from the doc tree to unblock the PR and CI.