Better clarity in docs about properties

[iamjoshua]

This is a minor issue, but the docs seem a bit too cavalier about the body type property:
http://brm.io/matter-js/docs/classes/Body.html

It's included with a bunch of other editable properties that seem to imply that it can be modified at will. However, it seems that this attribute absolutely should not be changed since the Composite class depends upon it:

matter-js/src/body/Composite.js

Line 83 in a5b535d

switch (obj.type) {

I just spent some time trying to figure out why Composite.remove stopped working after some recent changes that I made. I have many different object types in my project, so seeing this field in the docs I happily changed it to match...

It might be nice to add a Readonly header to properties like these in the docs.

Just some feedback. Overall, I've found working with Matter to be a great experience!