document thread safety for spectator

[brharrington]

Most usage should be thread safe, but it isn't clearly called out anywhere, or at least is easy to miss.

[brharrington]

Some projects such as caffeine indicate this with the findbugs jsr305 annotations. However this would add a dependency for spectator-api which we do not want. Also the package names would cause clashes in jdk9+.

[mohit975]

Hi