The GTK_TEXT_WINDOW_PRIVATE enumeration value is really *not* private.
Internally, it's used as a simple "invalid value" marker, and
application and library developers are supposed to use it as such in
their own code.

Let's just document it, and since the GtkTextView documentation and
internals go a long way to state the fact that it should not be used as
an argument value, let's add some pre-condition checks as well.

This commit fixes GtkSourceView's use of GTK_TEXT_WINDOW_PRIVATE as
default value for a GObject property that was broken by the change in
glib-mkenums to honor the `/*< public >*/` and `/*< private >*/`
trigraphs.