child_process.exec doc is inconsistent with actual implementation

[misterdjules]

child_process.exec' documentation states that the stdout and stderr parameters are instances of Buffer by default, but it's actually not the case. They are instances of Buffer only if { encoding: null} is passed as options.

The documentation of v0.12.x should be fixed to reflect this behavior.

The original intent was that stdout and stderr parameters would be instances of Buffer. If that still holds true, then the change in behavior should be done in master post v0.12.x.

Adding to milestone 0.12.2 as it's a quick change.

[tyleranton]

@misterdjules I can grab this.

[misterdjules]

@tyleranton That's great 👍 Please do and submit a PR when you're ready. Also feel free to ping me on #libuv if you need help. Thank you!