Explain "x < 0 && return 1" idiom

In the section on Conditional Evaluation, highlight Short-Circuit Evaluation as an alternative for very short if-statements.
In the section on Short-Circuit Evaluation, explain the idiom of "condition && statement", "condition || statement". 
Note that I don't know how this "doctest" stuff works, pls check my submission for correctness (I've copied/pasted straight from a Julia REPL).
Merge pull request #6336 from fabianlischka/patch-2

Explain "x < 0 && return 1" idiom