[thread.stoptoken] Index missing pieces in <stop_token>

[frederick-vs-ja]

Entities the std namespace:

• nostopstate_t,
• nostopstate,
• stop_callback_for_t,
• never_stop_token,
• inplace_stop_token,
• inplace_stop_source,
• inplace_stop_source, and
• inplace_stop_callback.

Member alias templates:

• All callback_type alias templates.

Member functions:

• All member functions shown in the synopses, except for
  • defaulted operator==, and
  • the destructor of inplace_stop_callback, because it's doubtful whether it should be explicitly mentioned at all, see also [functions.within.classes].

Exposition-only member class callback-type is skipped for now.

Fixes #8358.

[jensmaurer]

Thanks!

I've taken the liberty to squash-merge.