Update docs for network interfaces

[toddself]

doc: note that only active network ifaces appear

A proposal to fix #498 by clarifying the documentation.

[bnoordhuis]

I'm inclined to accept this because changing os.networkInterfaces() behavior w.r.t. offline interfaces would constitute a rather large break with joyent/node. Can other contributors weigh in?

@toddself Can you update the commit log per the guidelines in CONTRIBUTING.md? Please wrap long lines at 80 columns. Thanks!

[mscdex]

Adding "down" interfaces might be a nice addition. It would also be nice to tell by an extra property whether it's "down" or not though, or maybe we could group them (e.g. os.networkInterfaces().online and os.networkInterfaces().offline)?

[toddself]

Sorry! Updated with 80 column breaks.

[bnoordhuis]

@toddself Can you update the commit log one more time, please? :-) The first line should be doc: short description <= 50 chars followed by one or two lines explaining the what and why of the change.

[toddself]

@bnoordhuis is that what you meant in your last comment? or do i need to do a git commit --amend -m and a force push?

[bnoordhuis]

@toddself Please --amend it, yes. git log doc/ if you want some examples of the template io.js uses for commit logs.

[toddself]

@bnoordhuis thank you for the assistance on this! I tried to follow the format of previous commits that resolve an issue in the list.

[bnoordhuis]

@toddself Thanks, that looks great! Landed in 8b1db9c.