logout signature added in header as per the documentation

It is listed in documentation as public but not available in .h files. Difficult to see if you use salesforce template for ios project (uses compiled .a version of lib)

Ref: http://forcedotcom.github.io/SalesforceMobileSDK-iOS/Documentation/SalesforceSDK/Classes/SFNativeRestAppDelegate.html#//api/name/logout