Add note about process and thread synchronisers

Author: agriyakhetarpal

zarr/sync.py

@@ -13,7 +13,9 @@ def __getitem__(self, item):
 
 
 class ThreadSynchronizer(Synchronizer):
-    """Provides synchronization using thread locks."""
+    """Provides synchronization using thread locks. This is disabled for
+    platforms that don't support threading, such as Pyodide.
+    """
 
     def __init__(self):
         self.mutex = Lock()
@@ -34,7 +36,8 @@ def __setstate__(self, *args):
 class ProcessSynchronizer(Synchronizer):
     """Provides synchronization using file locks via the
     `fasteners <https://fasteners.readthedocs.io/en/latest/api/inter_process/>`_
-    package.
+    package. This is disabled for platforms that don't support threading, such
+    as Pyodide.
 
     Parameters
     ----------