Until now, we've only been explicitly documenting the `_converse.api`
namespace and only considered the methods under it as forming the API
contract (which determines how we do semver releases).

It appears as if we've reached a point where trying to keep everything
under the `_converse.api` namespace no longer makes sense. Certain
methods are applicable to particular models and trying to shoehorn them
into the `_converse.api` namespace seems clunky and non-intuitive.

I've therefore decided to slightly refactor `sendMessage` to let it take
two simple parameters and to document it with JSDoc so that it's
presented as an API method, albeit only available on a chat model.

updates #1496
updates #1504