Updated detached rulesets and mixins as functions

This pull request is add things we discussed in https://github.com/less/less.js/issues/2085, https://github.com/less/less.js/issues/2083 and https://github.com/less/less.js/issues/2064 .

Added few details about detached rulesets:
* short introduction with simplest possible example,
* return values from detached rulesets,
* scoping - which variables are seen from inside detached rulesets.

Added few details about mixins as functions:
* they return also mixins,
* local scope protection.