add information about return codes to manpage

[Karl-Philipp Richter]

The manpage of `wdiff` doesn't contain any information about return codes which are crucial for working with the program.

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: wdiff 1.2.2-2
ProcVersionSignature: Ubuntu 4.10.0-26.30-generic 4.10.17
Uname: Linux 4.10.0-26-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.4-0ubuntu4.1
Architecture: amd64
CurrentDesktop: XFCE
Date: Wed Jul 5 03:15:17 2017
Dependencies:
 gcc-6-base 6.3.0-12ubuntu2
 libc6 2.24-9ubuntu2.2
 libgcc1 1:6.3.0-12ubuntu2
 libtinfo5 6.0+20160625-1ubuntu1
InstallationDate: Installed on 2015-12-12 (570 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
SourcePackage: wdiff
UpgradeStatus: Upgraded to zesty on 2017-05-01 (64 days ago)