DOC: Update documentation for InTemporaryDirectory to mark it as not thread-safe

Author: Jacob Roberts

nibabel/tmpdirs.py

@@ -51,6 +51,9 @@ def __exit__(self, exc, value, tb):
 class InTemporaryDirectory(TemporaryDirectory):
     """ Create, return, and change directory to a temporary directory
 
+    NOTE: This implementation is not thread-safe and should be used with
+    caution by external consumers of this library.
+
     Examples
     --------
     >>> import os