prctl.2: Document EINVAL error for PR_SET_PTRACER

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
diff --git a/man2/prctl.2 b/man2/prctl.2
index 9cb6b25..2cae666 100644
--- a/man2/prctl.2
+++ b/man2/prctl.2
@@ -812,6 +812,17 @@
 resource limit to be exceeded.
 .RE
 .TP
+.B EINVAL
+.I option
+is
+.BR PR_SET_PTRACER
+and
+.I arg2
+is not 0,
+.BR PR_SET_PTRACER_ANY ,
+or the PID of an existing process.
+.TP
+TP
 .B EPERM
 .I option
 is