Fix doc for (preprocess (per_file ...))
Fix doc for (preprocess (per_file ...))

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