doc: Remove mention of child.send 'track' option

Will be removed very soon.  No point making it public.
2013.01.18, Version 0.9.7 (Unstable)

* V8: Upgrade to 3.15.11.7

* npm: Upgrade to 1.2.2

* punycode: Upgrade to 1.2.0 (Mathias Bynens)

* repl: make built-in modules available by default (Felix Böhm)

* windows: add support for '_Total' perf counters (Scott Blomquist)

* cluster: make --prof work for workers (Ben Noordhuis)

* child_process: do not keep list of sent sockets (Fedor Indutny)

* tls: Follow RFC6125 more strictly (Fedor Indutny)

* buffer: floating point read/write improvements (Trevor Norris)

* TypedArrays: Improve dataview perf without endian param (Dean McNamee)

* module: assert require() called with a non-empty string (Felix Böhm, James Campos)

* stdio: Set readable/writable flags properly (isaacs)

* stream: Properly handle large reads from push-streams (isaacs)