doc: note that v8 options work with both dashes and underscores
Checklist
-
documentation is changed or added -
the commit message follows commit guidelines
Affected core subsystem(s)
doc
Description of change
see https://github.com/nodejs/node/pull/6377#issuecomment-215601789
This adds docs to the man page and online cli docs that v8 options can be used with either.
r=@nodejs/documentation