Document the deprecated BOOST_INTERPROCESS_USE_PTHREAD_AS_SYSTEMWIDE_THREAD_ID option

Author: igaztanaga

doc/interprocess.qbk

@@ -6869,7 +6869,8 @@ thank them:
   was based on pid_t+pthread_t identifiers, something incorrect (as pthread_t was not suppossed to identify a thread, even inside a process).
   In this version, several Unix-like systems use a particular and correct global thread identifier (`gettid()` on Linux, 
   `pthread_getthreadid_np` on FreeBSD, `pthread_threadid_np` on MacOS, `_lwp_self` on NetBSD, `getthrid` on OpenBSD, `thr_self` on Solaris,
-  `GetCurrentThreadId` on Cygwin, `gettid` on QNX, etc...).
+  `GetCurrentThreadId` on Cygwin, `gettid` on QNX, etc...). The programmer can define `BOOST_INTERPROCESS_USE_PTHREAD_AS_SYSTEMWIDE_THREAD_ID`
+  to go back to the old `pthread_t` based behaviour, but this option is deprecated and will be removed in a few releases.
 
 * Fixed bugs:
    * [@https://github.com/boostorg/interprocess/issues/242 GitHub #242 (['"Cygwin compatibility issues"])].