Document thread-safety more clearly

[alecbz]

I imagine that it's safe to issue Get/Set/Fetch operations on the same cache from multiple threads, given the package is "aimed at high concurrency", and that the Clear method documents the opposite explicitly.

But if that's correct, might be good to call it out explicitly. Something like:

Unless otherwise noted (e.g., for Clear), methods on caches are thread-safe.

?

[karlseguin]

Sure
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