Add documentation about included WiX extensions

[volks73]

The WixUIExtension and WixUtilExtension are both always included in the execution of the compiler (candle.exe) and linker (light.exe) for the create/default subcommand. However, there is no mention of this fact in the documentation. So, details to this affect would reduce confusion.

[volks73]

Information about the default inclusion of both extensions has been added to the module-level documentation for the subcommand as of 8263eb8.