doc: note ability to require files within packages

Adding note to api/modules.markdown about ability to
load arbitrary path from modules from node_module path
by adding reference to path after module name in call
to require()

PR-URL: https://github.com/joyent/node/pull/7794


Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Fedor Indutny <fedor@indutny.com>
Reviewed-by: Timothy J Fontaine <tjfontaine@gmail.com>