spec to describe 'bonus' methods

[meowsbits]

Is your feature request related to a problem? Please describe.
The existing spec describes a subset of actually-available methods for go-ethereum.

Describe the solution you'd like

• rpc.discover_all describes all methods that are actually available for a given client instance.
• rpc.describe
• ...

Up for grabs: thoughts on a best name for this? Maybe an optional param for rpc.discover?

Describe alternatives you've considered
Client-specific methods like geth.discover. Downside is that they're not (necessarily) spec'd.

Additional context

// @multi-geth v1.9.0 console
> Object.keys(rpc.discover().methods).length
45
> Object.keys(eth).length
48