Short-circuit logic used as control flow should only be used on a single line

This came up over at https://github.com/invenia/LibPQ.jl/pull/197#discussion_r492802769

e.g. for && and ||

# Yes
if last_log == curr
    debug(LOGGER, "Consuming input from connection $(jl_conn.conn). Stand by for landing.")
end

# No, over line limit:
last_log == curr && debug(LOGGER, "Consuming input from connection $(jl_conn.conn). Stand by for landing.")

# No, use an `if` conditional:
last_log == curr &&
    debug(LOGGER, "Consuming input from connection $(jl_conn.conn). Stand by for landing.")

(aside: we may want to use a different example in the guide, given #59 is an open question)

This is consistent with out current advice on ternary conditionals:

Ternary operators (?:) should generally only consume a single line

i.e.

# Yes:
foobar = if some_big_long_really_long_expr_here_long == 2
    barrrr_more_long
else
    bazzz_also_not_short
end

# No:
foobar = some_big_long_really_long_expr_here_long == 2 ? barrrr_more_long : bazzz_also_not_short

Unlike ternary conditionals ?:, chaning short-circuit logic as conditionals is fine e.g. this is okay

is_red(x) || is_blue(x) || is_yellow(x) && println("It's a primary colour!")

Start by reading the guide's current ternary-operator advice and the surrounding control-flow guidance. Update the guide to state that short-circuit control flow should stay on one line, include the proposed examples, and consider whether the example should change in light of #59.】【。