Add a subsection for function types in typespecs docs

[1pakch]

Makes it easier to find the syntax used for typing anonymous functions which is not mentioned explicitly in the Getting Started guide and is impossible to find by searching for "function types".

I would update the guide as well by adding a paragraph mentioning function types and providing an example.

[josevalim]

@ilya-kolpakov I have pushed a commit to move this to literals and then I realized that we do mention those. That said, I don't think we should move them to separate section because they are literals after all. For example, should we move maps to a separate section because folks cannot find them?

If the issue is discovery, then we can find ways to improve it, but organization wise this is correct. Thank you.

[umamaistempo]

I am not sure this change is meaningful as they already exists in literals

[1pakch]

Discovery is indeed the issue. I spent quite some time trying to figure this out and did not find what the syntax was until I posted on Elixir forum even if I had the right page open in the browser. If I google "typespecs function types" I do not get anything pointing at the right syntax.

I see your point if this documentation is targeted for developers. I would say it is probably quite difficult for elixir devs to imagine the experience of someone quite new to the language, so please do not underestimate that gap.

EDIT: I think it is preferable to have this syntax next to "function types" (or "function types" and "typespecs") together so it would be easier to find. I think "anonymous functions" does not do the trick.