[DOCS] form: Add {__currentTimestamp} to ApiReference

Add a usage of {__currentTimestamp} to the example in
the ApiReference.

The feature has been added at a later point and is only mentioned
in the concepts, but never in the reference.

Resolves: #94023
Releases: master, 10.4
Change-Id: I4e1568343a8fa56e41f6549b4beae292523b8754
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68920


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/form/Documentation/I/ApiReference/Index.rst

@@ -2506,6 +2506,8 @@ Usage within form definition
          whereClause:
            uid: 1
          databaseColumnMappings:
+           tstamp:
+             value: '{__currentTimestamp}'
            pid:
              value: 1
          elements: