docs: add note about metadata dedup and --no[ac]time, fixes #2518

docs/usage/notes.rst

@@ -50,6 +50,17 @@ a new repository when changing chunker params.
 For more details, see :ref:`chunker_details`.
 
 
+``--noatime / --noctime``
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You can use these ``borg create`` options to not store the respective timestamp
+into the archive, in case you do not really need it.
+
+Besides saving a little space for the not archived timestamp, it might also
+affect metadata stream deduplication: if only this timestamp changes between
+backups and is stored into the metadata stream, the metadata stream chunks
+won't deduplicate just because of that.
+
 ``--umask``
 ~~~~~~~~~~~