docs: clarify thread safety support for free-threaded CPython build

Author: techouse

docs/README.rst

@@ -7,8 +7,8 @@ use is supported as well.
 
 Do not mutate caller-owned input containers or shared callback state while an
 ``encode()`` or ``decode()`` call is in progress. This applies equally to the
-free-threaded CPython build, which is tested experimentally but does not change
-these mutation guarantees.
+free-threaded CPython build, which is supported and covered by the thread-safety
+test suite without changing these mutation guarantees.
 
 Decoding
 ~~~~~~~~