safe-cast spec change

As discussed with @ncannasse, the [safe-cast documentation](https://haxe.org/manual/expression-cast-safe.html) should be changed to reflect that safe-casts throw "some" exception, not necessarily one of type `String` as documented.

Open the linked safe-cast documentation at haxe.org/manual/expression-cast-safe.html and compare its wording with the issue. Update the documentation so it says safe-casts throw some exception rather than necessarily a String exception; the work is done when that statement accurately reflects the behavior.

Written by the indexing model from the issue text.