getpriority.2: Expand discussion of getpriority() return value

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

man2/getpriority.2

@@ -118,7 +118,13 @@ soft limit; see
 .BR getrlimit (2)
 for details.
 .SH RETURN VALUE
-Since
+On success,
+.BR getpriority ()
+returns the calling thread's nice value, which may be a negative number.
+On error, it returns \-1 and sets
+.I errno
+to indicate the cause of the error.
+Since a successful call to
 .BR getpriority ()
 can legitimately return the value \-1, it is necessary
 to clear the external variable