doc: update process.versions.modules documentation

This commit adds a description of `process.versions.modules`,
based on the comment in `src/node_version.h` lines 47-50.

PR-URL: https://github.com/nodejs/node/pull/9901


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>