[racerd] improve interface not thread safe report text
[racerd] improve interface not thread safe report text

Summary:
The text is ambiguous because it sounds as if it recommends annotating the current class as `ThreadSafe`, not the interface invoked.

Also, remove the not useful part "or using an interface known to be thread-safe" because developers don't know in general what interfaces Infer thinks are thread-safe.

Reviewed By: skcho

Differential Revision: D20675729

fbshipit-source-id: 9da438621

facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer