[ci skip] Describe precision + scale in migrations guide

Telling somebody that "precision sets the precision" is not very helpful.
Newbies want to know what precision is *for*, likewise with scale.

It's also a pair of fields where I frequently forget what exactly they represent, and I know I'm not alone in that.

A few words in the relevant guide would be helpful for both newbies and forgetful experienced rails devs alike.

Thus I've added a very brief description for each.