scripts/coccinelle: update for compatability with Coccinelle 0.2.4

For doubleinit.cocci, Coccinelle 0.2.4 requires a comma after ... in a
field list.  Coccinelle also now behaves gracefully when a definition is
provided for a virtual that doesn't exist, so there is no need for the
semantic patch code to check for this case.

Updated the documentation to reflect the fact that the best results will
now be obtained with Coccinelle version 0.2.4 or later.

Signed-off-by: Julia Lawall <julia@diku.dk>