option --prof documentation help added

Hi, I trying to add prof documentation help on node_options.cc file, but when I test running show the following error:

../src/node_options.cc:405:35: error: no member named 'syntax_prof_only' in 'node::EnvironmentOptions' &EnvironmentOptions::syntax_prof_only);

So I don't know if I have to add 'syntax_prof_only' somewhere before.

Thaks for you help

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines