Bug#708576: Improve version-substvar-for-external-package description

[Joachim Reichel <reichel@debian.org>]

Package: lintian
Version: 2.5.10.4
Severity: wishlist

The version-substvar-for-external-package error is also triggered if the 2nd
package is simply misspelled. See for example the current lintian report for
cgal 4.2-2.

For Suggests: or Recommends: relationships such a misspelling can easily go
undetected. Please improve the description for this error to include this
cause, too.

Thanks,
  Joachim

-- System Information:
Debian Release: 7.0
  APT prefers stable-updates
  APT policy: (810, 'stable-updates'), (800, 'stable'), (700, 'testing')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.7-trunk-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash