Fix invalid HTMLForm::setValidationErrorMessage doc

Fix invalid HTMLForm::setValidationErrorMessage doc

The method has not actually supported scalar or single-level array
arguments for a while now; according to Michael Große, this ability was
lost in Ibb17bb61ac. No one seems to have noticed (indeed, MediaWiki
code search finds no callers at all [1]), so instead of trying to fix
it, let’s just update the documentation.

We can also remove a cast to array in trySubmit(), because if the value
wasn’t already an array, then converting it to a single-element array
and then trying to spread its only element into $hoistedErrors->fatal()
arguments still fails (“only arrays and Traversables can be unpacked”).

[1]: https://codesearch.wmflabs.org/search/?q=setValidationErrorMessage

Change-Id: I8c292ec62ef4aec89217e86a75d7f2e88111f43f