largefiles: add comment about non-atomic working directory

When updating the working copy with a normal 'hg update', we also
don't use atomic writes.

hgext/largefiles/lfutil.py

@@ -212,6 +212,8 b' def copyfromcache(repo, hash, filename):'
     if path is None:
         return False
     util.makedirs(os.path.dirname(repo.wjoin(filename)))
+    # The write may fail before the file is fully written, but we
+    # don't use atomic writes in the working copy.
     shutil.copy(path, repo.wjoin(filename))
     return True