Update phpdoc.dist.xml loading issue

When ran phpdoc without parameters it can't load phpdoc.dist.xml directly and show a "No parsable files were found" Exception.
Change userConfig file path search to take dist even if the user do not give a specific parameter.
This will throw an error if both $customUserconfigPath and $defaultUserConfigPath contain a non-existing path; at the very least should a is_readable check be added to the if-statement below (in addition to the null check)