[2.7][Console][Table] Add doc about colspan/rowspan + multiple header lines.

[aitboudad]

PR: symfony/symfony#13438

[javiereguiluz]

Here you can find a blog post about this feature that could be use as inspiration for the documentation: http://symfony.com/blog/new-in-symfony-2-7-advanced-table-layouts-for-console-commands

[wouterj]

This should be added to components/console/helper/table