Improve library documentation

[mniewrzal]

Jeff: so a thing i noticed from doing a lot of homework review is that users of the library tend to seem confused about when to parse/create accesses, open projects, etc. and how long they should be stored. for example, a common thing i've seen is, for every request, parsing the access, opening the project, doing the operation, and closing the project. sometimes this even includes doing the key derivation from the api key and password. maybe we should clarify this in the docs?

Fadila: And maybe also explain more specifically how to set expiration time and mention that it won't be accessible after that time (also noticed during homework reviews that it does not seem clear).

[pataquets]

FYI: docs-related issue #127