relnotes: document that `hg extdiff` also got -r replaced by --from/--to

Differential Revision: https://phab.mercurial-scm.org/D9628

relnotes/next

@@ -20,8 +20,8 b''
  * `hg strip`, from the strip extension, is now a core command, `hg
    debugstrip`. The extension remains for compatibility.
 
- * `hg diff` now supports `--from <rev>` and `--to <rev>` arguments as
-   clearer alternatives to `-r <revs>`. `-r <revs>` has been
+ * `hg diff` and `hg extdiff` now support `--from <rev>` and `--to <rev>`
+   arguments as clearer alternatives to `-r <revs>`. `-r <revs>` has been
    deprecated.
 
  * The memory footprint per changeset during pull/unbundle