[thread] Exposition-only formatting for private members

Affected sections and exposition-only identifiers:

[thread.jthread.class]
- _`ssource`_

[atomics.ref.generic]
- _`ptr`_

[util.smartptr.atomic.shared]
- _`p`_

[util.smartptr.atomic.weak]
- _`p`_

[thread.lock.guard]
- _`pm`_

[thread.lock.scoped]
- _`pm`_

[thread.lock.unique]
- _`pm`_
- _`owns`_

[thread.lock.shared]
- _`pm`_
- _`owns`_

[thread.sema.cnt]
- _`counter`_

[thread.latch.class]
- _`counter`_

[thread.barrier.class]
- _`completion`_

[futures.future.error]
- _`ec_`_

Drive-by changes:

[atomics.ref.float]
- Use `\placeholder` for _`floating-point-type`_

Author: frederick-vs-ja