Add comments to code

[jasonm23]

All functions and macros should have a docstring, to aid developers using the library.

A few I don't fully understand enough to document myself.

• [x] smt/deftype
• [x] smt/get
• [x] smt/t-visible-rows
• [x] smt/w-width-default
• [x] smt/combine-styles

(I'll have a crack at them though.)

[sabof]

I've added some comments on develbranch

[jasonm23]

Thanks I'll have a look in the morning

[jasonm23]

Looks ok, I will flesh them out a bit where necessary.