Bug#87510: how build dependencies are specified in policy

[Santiago Vila <sanvila@unex.es>]

+          When binary packages are on the so-called <tt>build-essential</tt>
+          list, the build-dependencies can be omitted. Please see
+          <ref id="pkg-relations"> for more information.

A package is build-essential when it's build-essential according to
the definition of build-essential. Any list, if exist, is only informational,
but not the definition of build-essential.

Please write "When binary packages are build-essential, the
build-dependencies can be omitted" instead.