fork(2): Add information about fork() in multi-threaded processes.

There is nothing about pthread_atfork(3) or extensions like calling
malloc(3) in the child process as this may be unreliable or broken.

lib/libc/sys/fork.2

@@ -73,6 +73,17 @@ are set to 0; see
 .It
 All interval timers are cleared; see
 .Xr setitimer 2 .
+.It
+The child process has only one thread,
+corresponding to the calling thread in the parent process.
+If the process has more than one thread,
+locks and other resources held by the other threads are not released
+and therefore only async-signal-safe functions
+(see
+.Xr sigaction 2 )
+are guaranteed to work in the child process until a call to
+.Xr execve 2
+or a similar function.
 .El
 .Sh RETURN VALUES
 Upon successful completion,
@@ -124,6 +135,7 @@ There is insufficient swap space for the new process.
 .Xr rfork 2 ,
 .Xr setitimer 2 ,
 .Xr setrlimit 2 ,
+.Xr sigaction 2 ,
 .Xr vfork 2 ,
 .Xr wait 2
 .Sh HISTORY