Coccinelle: Update the documentation

 - The new default mode is 'report'.
 - The available modes are detailed a bit more.
 - Some information about the use of spatch options are
   also given concerning the use of indexing tools.

Signed-off-by: Nicolas Palix <nicolas.palix@imag.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>