userfaultfd.2: Add details for UFFDIO_WAKE

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

man2/userfaultfd.2

@@ -365,6 +365,27 @@ or an error in the same manner as
 .TP
 .BI "UFFDIO_WAKE 	struct uffdio_range *" argp
 Wake up the thread waiting for page-fault resolution.
+.\" FIXME: Need more detail here. What is the purpose of the
+.\" 'struct uffdio_range *' argument? 
+
+This
+.BR ioctl (2)
+operation returns 0 on success.
+On error, \-1 is returned and
+.I errno
+is set to indicate the cause of the error.
+Possible errors include:
+.RS
+.TP
+.B EINVAL
+Either the
+.I start
+or the
+.I len
+field of the
+.I ufdio_range
+structure was not a multiple of the system page size.
+.RE
 .SH RETURN VALUE
 On success,
 .BR userfaultfd ()