Update detached-rulesets.md

Added scoping details and small introduction with simplest possible example.
Update mixins-as-functions.md

Added details about scope protection and information about return mixins.
Update mixins-as-functions.md

fixed typo
Update detached-rulesets.md
Merge branch 'detached-rulesets-1' of https://github.com/SomMeri/less-docs into detached-rulesets-1

Conflicts:
	content/features/mixins-as-functions.md
Merge pull request #201 from SomMeri/detached-rulesets-1

Updated detached rulesets and mixins as functions