Updates to phetioDocumentation

[arouinfar]

In #99 I discovered that there were a few properties that would benefit from phetioDocumentation.

@jonathanolson here is the suggested text, please feel free to edit as necessary.

• [x] comboBoxMaterialProperty: Current material of the block. Changing the material will result in changes to the mass, but the volume will remain the same.
• [x] materialEnumProperty: Current material of the block. Changing the material will result in changes to the mass, but the volume will remain the same.
• [x] customDensityProperty: Density of the block when the material is set to “CUSTOM”.
• [x] massProperty: Current mass of the block. Changing the mass will result in changes to the material (and therefore density), but the volume will remain the same.
• [x] volumeProperty: Current volume of the block. Changing the volume will result in changes to the mass, but will not change the material or density.

[jonathanolson]

Implemented, can you verify?

[arouinfar]

@jonathanolson looks pretty good. I wrote the phetioDocumentation with the blocks in mind, but forgot about scale.massProperty and scale.volumeProperty. Would it be possible to remove the phetioDocumentation for the scale? Since the scale isn't adjustable, I think the phetioID provides a sufficient enough description of these properties.

[jonathanolson]

I believe that should be handled above, can you verify?

[arouinfar]

Looks good, thanks @jonathanolson!