added notes regarding cases when ngx.location.capture and ngx.location.capture_multi fail to capture subrequests' outputs.
added notes regarding cases when ngx.location.capture and ngx.location.capture_multi fail to capture subrequests' outputs.

yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx,yeahdongcn/SEnginx