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
043d7eb

os2: thread: remove unused detached thread support
f09937d

win32: thread: remove unused detached thread support
8675f08

threads: remove vlc_clone_detach()
a10ac09

Deprecate vlc_clone_detach()
c5f960f

include/vlc_threads.h

@@ -688,7 +688,7 @@ VLC_API void vlc_cancel(vlc_thread_t);
  * <b>not</b> joined.
 
  * \warning A thread cannot join itself (normally VLC will abort if this is
- * attempted). Also a detached thread <b>cannot</b> be joined.
+ * attempted).
  *
  * @param th thread handle
  * @param result [OUT] pointer to write the thread return value or NULL