doc: v8 functions as methods on v8

For consistency with other docs, show v8 functions as being on the v8
object.

PR-URL: https://github.com/nodejs/node/pull/6615


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>