Improve CSS property documentation

[jcfilben]

We should standardize the way we document properties that map to css properties. There are a few different potential scenarios that could happen:

• grommet prop maps 1 to 1 with css property
• grommet prop can accept some but not all of the options that could be passed to a css property
• grommet prop can accept all the options passed to a css prop in addition to other options
• grommet prop can accept some css property options and some additional options

We should take into consideration the above scenarios when designing a solution. We have also received feedback that it would be helpful to make it more clear when a grommet prop maps to a css property. So something that visually calls attention in these cases might be helpful.