CocoaPods: Instructions how to enable HealthKit (sample Podfile)

Also improves the log message when presumably the wrong framework target was used.

I have tested and verified that without the modified Podfile, HealthKit is *not* linked against, and the assert when trying to use HealthKit request categories is indeed triggered.

Accordingly, with the modified Podfile, the assert is not triggered, and HealthKit is not linked into the app at runtime. I have tested this with the default Debug and Release configurations that Xcode provides for project templates.