doc/pages/io.dox: writers don't set EOF flag at all.¹

1: Especially not ones that grow! Duh! If a write operation would
extend past the size of a non-growing stream, status should have
the fail flag instead.
doc/pages/io.dox: added note about dest buffer state from failed read.
utility: added data member pointer array_extent() variant.¹

1: Additionally: array_extent() now taked T const.