Trying to make the docs better

[saurabhnanda]

Please do not merge this yet - it is incomplete.

If you are open to such a change, I'll complete this.

[jakubfijalkowski]

I'd love to see this merged. :) hlibsass documentation isn't the best one, so every change is very welcome.

One slight request from me - explicitly state (and make it stand out) that hlibsass version works only with corresponding Libsass version and will break if used with other (recently, there was a couple of problems with hsass in this regard and because of that I am not sure if relying on external library is the go-to approach).