Fix: Empty @discussion blocks cause warnings with -Wdocumentation

Clang 3.2 (Xcode 5) adds support for the `-Wdocumentation` flag, which creates a incredibly helpful reminder to update documentation when methods change.

At the moment, `#include`ing Facebook SDK headers causes project warnings with this flag. This pull request fixes those warnings.

Fixes https://developers.facebook.com/bugs/566203590121364 .