fix presentation of `tuple_element` specializations

[zygoloid]

We have a variety of tuple_element specializations that use different presentation styles:

• Some give a definition of the specialization
• Others don't, and merely specify the ::type member
• Some use a Value: element for the type, others use a Type: element

We should do this properly: the specialization needs a specified definition, and a member typedef should be specified as actually being a type. (The current wording for several of these specializations completely lacks any suggestion that ...::type is a typedef-name, for example.)

[JohelEGP]

#2553, #2476 and https://wg21.link/LWG3166 are related.

[jensmaurer]

LWG 3166 is concerned with the undefined "Value:" element. Let's ignore this issue for now (since it's tracked elsewhere), and let's harmonize all tuple_element specialization as outlined by @zygoloid above.