Brackets around a parameter name means that it is optional. Here's some examples of parameter doc:
`@param left ...` means `left` is required.
`@param [left] ...` means `left` is optional.
`@param [left=20.0] ...` means that `left` is optional and if it is `undefined` the default value is `20.0`. 
You should look through the code for other examples. Change the doc here and throughout this file to make its consistent with the rest of Cesium.