Update wixl.vala to show it can take multiple input files, which it does in the test suite

this changes the help documentation so it is more obvious to the user that they can submit multiple files to the wixl command and it will combine them into a single .msi file. This is used in the testsuite. It is also important if one wants to merge a wxs file with another fragment wxs file? (perhaps created by wixl-heat?)