clarify short-circuit && and || docs (#56420)
This clarifies the docs to explain that `a && b` is equivalent to `a ? b
: false` and that `a || b` is equivalent to `a ? true : b`.
In particular, this explains why the second argument does not need to be
a boolean value, which is a common point of confusion. (See e.g. [this
discourse
thread](https://discourse.julialang.org/t/internals-of-assignment-when-doing-short-circuit-evaluation/122178/2?u=stevengj).)
