Elaborate on how style auto-prefixing works

[skirtles-code]

Closes #1059.

I've tried to give a little more detail about how the auto-prefixing of styles works. Specifically:

1. I've removed the mention of transform as that hasn't needed prefixing for ages.
2. I've tried to make it clearer that a prefixed style will only be included if it is required by the current browser. The previous wording may have led some people to expect to see multiple, prefixed properties, similar to what you might get in a CSS file. I think that was the underlying source of confusion in Vue failed to 'Auto-prefixing' ? #1059.