Update documentation for thread management

Added information about setting thread counts via environment variables.

Author: orena1

src/blosc2/core.py

@@ -892,6 +892,11 @@ def free_resources() -> None:
     temporary space.  You can use this function to release these
     resources when you are not going to use Blosc for a long time.
 
+    The number of threads can also be set via the ``BLOSC_NTHREADS`` environment
+    variable (e.g., ``export BLOSC_NTHREADS=1``). Additionally, you may want to set 
+    ``NUMEXPR_NUM_THREADS`` as well since numexpr is used under
+    the hood (e.g., ``export NUMEXPR_NUM_THREADS=1``).
+
     Examples
     --------
     >>> blosc2.free_resources()