Changes

Update documentation for the Thread module and deprecate some functions (#9419)

- Deprecate Thread.kill, Thread.wait_read, Thread.wait_write
  for reasons explained in the documentation comments.
- Update documentation comments for Thread functions.
- Deprecate ThreadUnix module, documented as deprecated since 2002
  (commit 0a47a75d56).
- In the manual, remove leftover mentions of the VM threads
  implementation; focus on the system threads implementation.
- In the manual, remove the documentation of ThreadUnix.

Closes: #9206
 (commit: 080ac74)