Could you please write a patch on Doc/library/tracemalloc.rst? It's really hard to review your raw comments in comments.

You should probably sign the contributor agreement.
http://www.python.org/psf/contrib/contrib-form/

"""
Is [clear_traces()] just a shortcut for stop();start()  ?  If so, please say so.
"""

It's almost the same. clear_traces() does not uninstall and reinstall hooks on memory allocators. I prefer to not give too much details in the high-level documentation.

"""
Line 558, similar to above:
      the new snapshots (``int``): ``0`` if the memory blocks have been
      allocated in the new snapshot.
->
      the new snapshots (``int``).  size_diff==size if the memory blocks do not appear in the old snapshot.
"""

This is not correct, size_diff is 0 if memory blocks are new, not size.