Change sphinx default role to "any" - #40

This PR changes the sphinx default role so that comments in single-backticks render as literal text and link(!) to the appropriate section of the docs if sphinx can find one.

This recommendation comes from the sphinx docs:

This role is a good candidate for setting default_role. If you do, you can write cross-references without a lot of markup overhead.

One downside is that sphinx can't find a target for some text it'll emit a warning:

docstring of opentelemetry.trace:9: WARNING: 'any' reference target not found: BlankSpan

but we can get around this by using double-backticks to specify the :code: role, which is what we were doing before this change anyway.