Add docstring for BVPFunction to bvp_types.md

[abhro]

The docstring for BVPFunction currently doesn't appear anywhere on the docs site.

Checklist

• Appropriate tests were added
  None applicable
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

This PR puts the docstring for the BVPFunction type in the context of the documentation website.