doc: note about custom inspect functions

See: https://github.com/nodejs/io.js/issues/1798

When an Object is printed in REPL, the actual representation can be
overriden by defining `inspect` method on the objects. This patch
includes a note about the same in the REPL documentation.

PR-URL: https://github.com/nodejs/io.js/pull/2142


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>