add a note about `preprocessing_deps` in preprocessing specification
add a note about `preprocessing_deps` in preprocessing specification

Signed-off-by: zapashcanon <1f0a51c36efaa0f44e4899c26d2028681997c8ea@ndrs.fr>

janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder