[AD] Patch: Docs corrections and clarifications

[Polybios <polybios@xxxxxxxxxx>]

Dear developers,

I'd like to submit this humble patch to the documentation. Basically, 
I've simply gone through all of it, correcting all the (spelling) 
mistakes I've noticed and adding some clarifications.
I've also added short and very basic introductory texts to three 
sections. These might not be perfect and I don't particularly mind if 
they will be included.

By the way, it would be nice if we could link to non-API 
headings/sections in a clean way (i.e. that will not only work with html 
output); especially a link from the ALLEGRO_EVENT_KEY_... events to the 
"Key codes" section is on my mind here.
But if I understand correctly, this is not possible with the current 
setup of the doc-generating scripts.

Kind regards,
Polybios