Reworked "Parent Selectors" section.

[#1901](https://github.com/less/less.js/issues/1901) reminded me of [#40](#https://github.com/less/less-docs/issues/40#issuecomment-32071441) so here we go.
I only collected typical examples and set initial structure up. The text is pretty raw there so I hope someone will find time to improve it.