Add info for functions

[eminence]

A while ago we added support for getting info about units and variables,

Now that we're getting some interesting functions, let's extend the info mechanism (and related decorators) to support functions

[sharkdp]

Yes — that would be great to have. Thank you for creating the ticket. It would also allow us to auto-generate the list of functions (https://numbat.dev/doc/list-functions.html) and to enrich it with more information.