getpriority.2: Improve description of setpriority() return value

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

man2/getpriority.2

@@ -126,10 +126,12 @@ to clear the external variable
 prior to the
 call, then check it afterward to determine
 if \-1 is an error or a legitimate value.
-The
+
 .BR setpriority ()
-call returns 0 if there is no error, or
-\-1 if there is.
+returns 0 on success.
+On error, it returns \-1 and sets
+.I errno
+to indicate the cause of the error.
 .SH ERRORS
 .TP
 .B EINVAL