Makefile: split off SPATCH_BATCH_SIZE comment from "cocci" heading

Makefile: split off SPATCH_BATCH_SIZE comment from "cocci" heading

Split off the "; setting[...]" part of the comment added in In
960154b9c17 (coccicheck: optionally batch spatch invocations,
2019-05-06), and restore what we had before that, which was a comment
indicating that variables for the "coccicheck" target were being set
here.

When 960154b9c17 amended the heading to discuss SPATCH_BATCH_SIZE it
left no natural place to add a new comment about other flags that
preceded it. As subsequent commits will add such comments we need to
split the existing comment up.

The wrapping for the "SPATCH_BATCH_SIZE" is now a bit odd, but
minimizes the diff size. As a subsequent commit will remove that
feature altogether this is worth it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>