Documentation: Impove CASE_DEFAULT documentation

[kevinsung]

What should we add?

From https://qiskit.org/documentation/stubs/qiskit.circuit.SwitchCaseOp.html:

The special value CASE_DEFAULT can be used to represent a default condition.

But it doesn't explain how to import CASE_DEFAULT and there is no hyperlink.

[jakelishman]

Yeah, it's meant to hyperlink to an entry right underneath this table of the control-flow ops, but I must have screwed up the Sphinx syntax, because that entry hasn't actually been build, so there's nothing to link to.

I'll fix it tomorrow, because I don't want to put up a new PR right now while we're in a CI concurrency crunch for the 0.25.0rc1 release today (or early tomorrow if we can't get through CI).

[jakelishman]

I've made #10484 that hopefully should make a bunch of improvements here.