docs(form-field): placeholder vs label behavior in various appearance modes needs to be better explained

[mmalerba]

Here's a writeup on stackblitz I created to explain to someone why floatLabel="never" doesn't do anything in standard, fill, or outline mode: https://stackblitz.com/edit/angular-material2-issue-xdhjap?file=app/app.component.html this needs to be better explained in the docs

[tomasdev]

Probably related: having floatLabel="never" currently doesn nothing.

https://angular-qvxcxp.stackblitz.io