Added documentation for IPAddressv4
Added documentation for IPAddressv4

Summary:
Tidied up the documentation for this class.

(Note: this ignores all push blocking failures!)

Reviewed By: Gownta

Differential Revision: D38866604

fbshipit-source-id: e84d8a52e9b58dadababaf279c671f6b236144ee

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly