doc: describe u-boot-test-release

The scripts u-boot-test-release is called at the end of testing.
Describe it.

Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/develop/py_testing.rst b/doc/develop/py_testing.rst
index 5ad7f57..deea048 100644
--- a/doc/develop/py_testing.rst
+++ b/doc/develop/py_testing.rst
@@ -389,6 +389,13 @@
 avoid the need for `u-boot-test-flash` to actually write U-Boot to flash, thus
 saving wear on the flash chip(s).
 
+u-boot-test-release
+'''''''''''''''''''
+
+When all tests for the board have been executed, this script is called.
+
+The board can be switched off now.
+
 Examples
 ''''''''