Include function signatures, arguments and keywords in docs

[rafaqz]

I was just trying to learn what functions are available in this package from the REPL, but its pretty hard to do.

Its common to include a method signature at the top of Julia docs.
Then its nice to list arguments and keywords describing what they do.

Something like this:

"""
    species_citation(name::String, [region::String]; kw...)

Species citations bla bla

# Arguments

- `name`: name of the foo
- `region`: region for the bar

# Keywords
...
"""

The square bracket is for optional arguments