document useful but undocumented property of /proc/.../wait

Reference: /n/sources/patch/applied/wait-length
Date: Fri Jun 14 11:21:08 CES 2013
Signed-off-by: charles.forsyth@gmail.com

--- /sys/man/3/proc	Fri Jun 14 11:20:56 2013
+++ /sys/man/3/proc	Fri Jun 14 11:20:53 2013
@@ -170,6 +170,10 @@
 a read of
 .B wait
 will block.
+If the file's length is non-zero
+(see
+.IR stat (2)),
+there is at least one wait record to read.
 It is an error for a process to attempt to read its own
 .B wait
 file when it has no children.