CPP1117 — Missing description related to union member modifier

In the specification an addition was made about the default argument when the union member has multiple legal discriminator values:

Union members associated with the "default" label and those associated with multiple labels may have multiple legal discriminator values. For these members, the modifiers (those that return void) have a second parameter which has the discriminator's type. This parameter has a default argument. For members associated with multiple labels, the value of the default argument is the value of the case label that appears first in IDL. For members associated with the default label, the value of the default argument is an implementation-defined value that selects the member.

There is nothing in the text that describes what happens when a discriminator value is passed which doesn't belong to the member, this is implicitly in the example code further down "u.obj(a, 2)", but this should be explicit in the text also, passing a not allowed discriminator should result in a BAD_PARAM exception

Add description of error handling for 2-parameter union modifier functions

To be consistent with the existing parts of the spec, make it clear that invalid parameters result in a BAD_PARAM exception.