Add a note under IMPLEMENTATION NOTES about the behavior of sendfile()

in the threaded library.

lib/libc/sys/sendfile.2

@@ -103,6 +103,33 @@ is returned.
 The FreeBSD implementation of
 .Fn sendfile
 is "zero-copy", meaning that it has been optimized so that copying of the file data is avoided.
+.Pp
+In the non-threaded library
+.Fn sendfile
+is implemented as the
+.Va sendfile
+syscall.
+.Pp
+In the threaded library, the
+.Va sendfile
+syscall is assembled to
+.Fn _thread_sys_sendfile
+and
+.Fn sendfile
+is implemented as a function which locks
+.Fa fd
+for reading and
+.Fa s
+for writing, then calls
+.Fn _thread_sys_sendfile .
+If the call to
+.Fn _thread_sys_sendfile
+would block, a context switch is performed.  Before returning,
+.Fn sendfile
+unlocks
+.Fa fd
+and
+.Fa s .
 .Sh RETURN VALUES
 Upon successful completion,
 .Fn sendfile