select.2: Consolidate historical glibc pselect() details under one subhead

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

man2/select.2

@@ -496,16 +496,6 @@ Portable programs may wish to check for
 .B EAGAIN
 and loop, just as with
 .BR EINTR .
-.PP
-Under glibc 2.0,
-.I <sys/select.h>
-gives the wrong prototype for
-.BR pselect ().
-Under glibc 2.1 to 2.2.1, it gives
-.BR pselect ()
-when
-.B _GNU_SOURCE
-is defined.
 .\"
 .SS The self-pipe trick
 On systems that lack
@@ -630,6 +620,22 @@ See
 .BR sigprocmask (2)
 for a discussion of the difference between the kernel and libc
 notion of the signal set.
+.\"
+.SS Historical glibc details
+.PP
+Glibc 2.0 provided an incorrect version of
+.BR pselect ()
+that did not take a
+.I sigmask
+argument.
+.PP
+In glibc versions 2.1 to 2.2.1,
+one must define
+.B _GNU_SOURCE
+in order to obtain the declaration of
+.BR pselect ()
+from
+.IR <sys/select.h> .
 .SH BUGS
 POSIX allows an implementation to define an upper limit,
 advertised via the constant
@@ -661,12 +667,6 @@ According to POSIX, any such file descriptor that is specified in one
 of the sets should result in the error
 .BR EBADF .
 .PP
-Glibc 2.0 provided a version of
-.BR pselect ()
-that did not take a
-.I sigmask
-argument.
-.PP
 Starting with version 2.1, glibc provided an emulation of
 .BR pselect ()
 that was implemented using