It would be great if the section tag could accept a |placeholder parameter, like the field that is very similar to it, i.e textarea, and like the free text standard input.
Current functionality of section tags is documented at
https://www.mediawiki.org/w/index.php?title=Extension:Page_Forms/Defining_forms&oldid=3033675#'section'_tag
See also: T229151
@ahmad: This task seems to cover several requests. Note that https://www.mediawiki.org/wiki/How_to_report_a_bug asks to "Include only one problem per task", so this task might be closed as invalid as it's very complicated to track progress and status for each of the requests in this single task.
Sorry for the delay in responding. I believe the "placeholder" parameter has been supported for section tags for a long time, but it just wasn't listed in the documentation. I just added it to the documentation - sorry about that.