Updated notes on thread-safety.

README.md

@@ -56,12 +56,9 @@ Disable it again and clear any existing statistics:
 ## Thread Safety
-The C extension uses a mutex to ensure the various methods provided by this Gem
-can be used in different threads simultaneously. Each call to
-`Allocations.to_hash` returns a new Hash containing a copy of the current
-statistics (instead of just referring to a single global Hash).
-
-Do note that calling `Allocation.start` and `Allocations.stop` affects _all_
+This C extension does not use any internal locking and instead relies on the GIL
+to prevent multiple Ruby threads from accessing the allocation data in parallel.
+Note that calling `Allocation.start` and `Allocations.stop` also affects _all_
 running threads instead of only the current thread.
 ## License