validation_error documentation

[matwey.kornilov@…]

validation_error constructor signature was changed from

validation_error(const std::string &);

in boost 1.41.0

to

validation_error(kind_t, const std::string & = "", const std::string & = "");

in boost 1.42.0+

validation_error isn't backward compatible now. This way "Custom Validators" section in html-documentations has an wrong example that uses 'validation_error(const std::string &)' signature. And there aren't any examples to use new version of validation_error.

http://www.boost.org/doc/libs/1_43_0/doc/html/program_options/howto.html#id1421830

[Marshall Clow]

This is a duplicate of #3992.