Fix docs about variable name resolving

I've tried to make docs more precise.

The problem is in difference between docs and specs. Namely, in spec https://github.com/mustache/spec/blob/master/specs/sections.yml#L67 variable name is searched not only in the current context but in all its parents too. But the docs say that only current context will be checked.

Here is the discussion of the problem: https://github.com/mustache/mustache.github.com/issues/30