doc: event: Correct EVT_DM_POST_INIT_F description

EVT_DM_POST_INIT_F only works in U-Boot proper, not SPL.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

Polish the wording a bit

Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>