The documentation previously implied that they could. That’s not really
true though: they can only fail if preconditions fail, i.e. they’re
passed invalid input. That’s a programmer error, which is not something
we want to encourage people to check for at runtime (e.g. by dynamically
checking for a 0 return value).

Signed-off-by: Philip Withnall <pwithnall@gnome.org>