v4: Responsive tables not clearly documented

[TheZoker]

In the migrating guide there is this one line:

Responsive tables no longer require a wrapping element. Instead, just put the .table-responsive right on the <table>.

But on the docs the example is still like in v3.
http://v4-alpha.getbootstrap.com/content/tables/#responsive-tables

Also the "not-wrapping" version does not really work:

There is always empty space in the table

[mdo]

This was addressed with #21208, which fixed #18612.