These are included in the product guide with every major release.
Here is the one for 7.7: McAfee KnowledgeBase - Web Gateway 7.7.0 Product Guide see page 703.
Additionally, descriptions are included in the help pages built into MWG, and also there is a small description snippet included with the property when you're building the criteria.
I'm assuming you mean properties (like client.ip) when you say variables. When you say parameters, some properties need parameters to evaluate. Like URL.Categories<Default>, "Default" in this case is a "settings container" which tells MWG how to perform the category lookup (i.e. use the cloud, dont use the cloud, look for embedded URLs, enable dynamic classification, etc... ).
Each of the different settings are covered in the product guide as well, URL Filtering for example is under page 358.