doc: v8 options can use either `_` or `-`

This adds docs to the man page and online cli docs that v8 options can
be used with either dashes or underscores.

Refs: https://github.com/nodejs/node/pull/6377#issuecomment-215601789
PR-URL: https://github.com/nodejs/node/pull/6532


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>