Top-level API should list possible args/kwargs explicitly

[sentrivana]

Problem Statement

Some of the top level API docs only mention **kwargs without any further explanation, so people have to dive into the code themselves.
- e.g. start_span, capture_message

Solution Brainstorm

See if we can expand the kwargs at least in top-level API to the actual arguments.

• When it comes to user-facing API, user-friendliness should be key. It'd also make code completion possible in IDEs.
• See if there's a smart way to do this without duplicating code. If not, I guess we can live with some code duplication in the name of making our user-facing API more usable.
• If not possible, alternatively we should at least include something in the docstring to point the user in the right direction.

See also #2196