RelNotes/2.21: tweak "--date=auto" mention

In the feature that was eventually committed, "--date=auto" doesn't do
anything. It was generalized to "--date=auto:<format>".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.21.0.txt

@@ -77,9 +77,9 @@ UI, Workflows & Features
  * A new date format "--date=human" that morphs its output depending
    on how far the time is from the current time has been introduced.
-   "--date=auto" can be used to use this new format when the output is
-   going to the pager or to the terminal and otherwise the default
-   format.
+   "--date=auto:human" can be used to use this new format (or any
+   existing format) when the output is going to the pager or to the
+   terminal, and otherwise the default format.
 Performance, Internal Implementation, Development Support etc.