Update affix docs to include mention of potential parent collapse

[koichirose]

I noticed affix uses this css declaration in 'docs.css' and if you don't include it the affixed sidebar will collapse:

/* prevent column from collapsing when affixing */
.span3 {
  min-height: 1px;
}

You should warn the users and save us some time :)

[mdo]

It's not nonstandard, but yes, it is required and I'll make a mention in the docs for 2.1.1.

[richardp-au]

Why not simply make this part of the core .spanXX classes? I can't see how this could affect anyone and then they don't have to do it manually when using affix.