vlc_threads: remove mention of detached thread

The support for detached threads has been removed completely. There is no need to document then in vlc_join now.

See the following commits:

android: thread: remove unused detached thread support 043d7ebf

os2: thread: remove unused detached thread support f09937dc

win32: thread: remove unused detached thread support 8675f083

threads: remove vlc_clone_detach() a10ac09d

Deprecate vlc_clone_detach() c5f960ff