nice.2: Rework discussion of nice() return value and standards conformance

Make the text a little clearer. In particular, clarify that the
raw system call (still) returns 0 on success.

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

man2/nice.2

@@ -84,7 +84,7 @@ resource limit in
 .BR setrlimit (2).)
 .SH CONFORMING TO
 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
-However, the Linux and (g)libc
+However, the raw system call and (g)libc
 (earlier than glibc 2.2.4) return value is nonstandard, see below.
 SVr4 documents an additional
 .B EINVAL
@@ -98,23 +98,21 @@ the addition of the "autogroup" feature in Linux 2.6.38 means that
 the nice value no longer has its traditional effect in many circumstances.
 For details, see
 .BR sched (7).
-
-SUSv2 and POSIX.1 specify that
-.BR nice ()
-should return the new nice value.
-However, the Linux system call and the
-.BR nice ()
-library function provided in older versions of (g)libc
-(earlier than glibc 2.2.4) return 0 on success.
-The new nice value can be found using
-.BR getpriority (2).
 .\"
 .SS C library/kernel differences
-Since glibc 2.2.4,
+POSIX.1 specifies that
 .BR nice ()
-is implemented as a library function that calls
+should return the new nice value.
+However, the raw Linux system call returns 0 on success.
+Likewise, the
+.BR nice ()
+wrapper function provided in glibc 2.2.3 and earlier returns 0 on success.
+
+Since glibc 2.2.4, the
+.BR nice ()
+wrapper function provided by glibc provides conformance to POSIX.1 by calling
 .BR getpriority (2)
-to obtain the new nice value to be returned to the caller.
+to obtain the new nice value, which is then returned to the caller.
 With this implementation,
 a successful call can legitimately return \-1.
 To reliably detect an error, set