Strange behaviour of the alternative operator //

Using JQ 1.5 I've problems understanding the output of the alternative operator // in a specific situation.

Given this input:

{
    "a": 42,
    "b": false,
    "c": null
}

I expected the expression (.a, .b, .c, .d) // -1 to return this:

42
-1
-1
-1

But in fact it returns this instead:

42

If I replace // with another operator like < then I indeed get four results instead of just one (the actual results are not important here, just their number):

> jq '(.a, .b, .c, .d) < -1' input.json
false
true
true
true

Note: The expression (.a, .b, .c, .d) | . // -1 returns the expected output. This is not the question. I'd like to know why the initial expression does not work.

The 1.5 documentation is, to put it bluntly, just wrong.

The jq FAQ provides this succinct and useful summary:

"A // B" either produces the truthy elements of A if there are any,
or else the entire stream B.