Documentation style guidelines

Some guidelines on the style we should adopt/apply suggested by @Bananattack :

- use consistent formatting;
- any new information / rewrites should be written clearer than the original;
- keep things concise and short;
- don't repeat information like register/mode descriptions, reference/backlink to existing information instead;
- present things as clear rules/definitions first, then explain or elaborate after.

Related issues:
#55