Add optional function annotations

[maartenvanvliet]

This PR adds the possibility to specify additional function annotations that will be rendered by ExDoc. This is at the function level what #1430 does for modules.

My use case is that a bunch of functions are generated at compile time and I want to mark them as such using @doc tags: [:generated]

This would be the result:

[josevalim]

Thanks @maartenvanvliet! Unfortunately I am a bit reserved about using :tags for this, given that tags in EDoc mean something else.

One thing we could do is to introduce a annotations_for_functions, which receives the metadata documentation and return a list. In the same way groups_for_functions and similar also handle functions.

[maartenvanvliet]

I've added the annotations_for_functions config option. It's invoked with the metadata. It's not invoked for callbacks and types though, that could be added but then the function would probably need a new name :)

[josevalim]

Apparently groups_for_functions is called for callbacks, so it may be a good idea to do the same here.

We can either keep it annotations_for_functions or rename it to annotations_for_entries or annotations_for_docs (given they are both declared with @doc). Any suggestions?

Then I will rename groups_for_functions later.

[josevalim]

Oh, and please rebase could I made some changes to prepare the unification. :D Sorry!˜

[maartenvanvliet]

I've gone with annotations_for_docs

[josevalim]

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