semantics of substring()

I don't understand one of the examples given for fn:substring() in the F and
O documentation [7.4.3]. The description of the function's behaviour says,
"If $startingLoc is zero or negative, the substring includes characters from
the beginning of the $sourceString." I take this to mean that

    substring( "12345", 0, 3 )

is equivalent to

    substring( "12345", 1, 3 )

which I understand evaluates to "123".

The example shown in the documentation however returns "12", and Saxon
echoes this. What am I misunderstanding?

TIA,
Howard