coccicheck: refer to Documentation/coccinelle.txt and wiki

Refer to the Documentation/coccinelle.txt and supplemental documentation
on the wiki:

https://bottest.wiki.kernel.org/coccicheck

This page shall always refer to the linux-next iteration of scripts/coccicheck.

v4: only refer to the wiki as supplemental documentation, and also
    update Documentation/coccinelle.txt.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Acked-by: Nicolas Palix <nicolas.palix@imag.fr>
Signed-off-by: Michal Marek <mmarek@suse.com>