expand on the stability by default of sort functions (and small fixes)

As per @StefanKarpinski's [request](https://github.com/JuliaLang/julia/issues/939#issuecomment-53304162).  Explain more the rationale behind the choice of default sorting algorithms, and give explicit alternatives for when efficiency is paramount. As done by a non-english beginner, feel free to take over (or comment) this PR in whatever ways. 

I wanted to mention in the docs the existence of the SortingAlgorithms package, would this be OK?

Also, there is much redundance around `by`/`lt`/`rev`/`alg` ("according to the specified order"...) which could be factorized out, and the question whether the first verb in the description of a function is not consistently decided (e.g. "return" vs "returns", "sort" vs "sorts").