texinfo: fix an imprecise comment

texinfo: fix an imprecise comment

In GNU make, '$^' used in a recipe contains every dependency for the
target, even those not declared when the recipe is read; for example,
on:

    all: foo1; @echo $^
    all: foo2

"make all" would output "foo1 foo2".  This is not Automake's fault,
as some comments incorrectly stated.  See also:
<http://lists.gnu.org/archive/html/help-make/2012-05/msg00029.html>

* lib/am/texinfos.am: Fix those incorrect comments, and document the
GNU make behaviour described above.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>