Update docstring verbiage per PR review

Co-authored-by: Matthew Brett <[email protected]>

Author: jacobdr

nibabel/tmpdirs.py

@@ -51,8 +51,11 @@ 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.
+    Notes
+    ------
+    As its name suggests, the class temporarily changes the working
+    directory of the Python process, and this is not thread-safe.  We suggest
+    using it only for tests.
 
     Examples
     --------