sigaction.2: Add discussion of rt_sigaction(2)

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/sigaction.2

@@ -48,7 +48,7 @@
 .\"
 .TH SIGACTION 2 2015-01-22 "Linux" "Linux Programmer's Manual"
 .SH NAME
-sigaction \- examine and change a signal action
+sigaction, rt_sigaction \- examine and change a signal action
 .SH SYNOPSIS
 .nf
 .B #include <signal.h>
@@ -836,6 +836,36 @@ See
 .BR signal (7)
 for a list of the async-signal-safe functions that can be
 safely called inside from inside a signal handler.
+.\"
+.SS C library/kernel ABI differences
+The original Linux system call was named
+.BR sigaction ().
+However, with the addition of real-time signals in Linux 2.2,
+the fixed-size, 32-bit
+.IR sigset_t
+type supported by that system call was no longer fit for purpose.
+Consequently, a new system call,
+.BR rt_sigaction (),
+was added to support an enlarged
+.IR sigset_t
+type.
+The new system call takes a fourth argument,
+.IR "size_t sigsetsize" ,
+which specifies the size in bytes of the signal sets in
+.IR act.sa_mask
+and
+.IR oldact.sa_mask .
+This argument is currently required to have the value
+.IR sizeof(sigset_t)
+(or the error
+.B EINVAL
+results).
+The glibc
+.BR sigaction ()
+wrapper function hides these details from us, transparently calling
+.BR rt_sigaction ()
+when the kernel provides it.
+.\"
 .SS Undocumented
 Before the introduction of
 .B SA_SIGINFO