Improve description of getXMLTree

[sKarki999]

0031236: Improve description of getXMLTree https://mantis.ilias.de/view.php?id=31236

The existing SOAP documentation for getXMLTree() doesn't explain how its parameters work, especially the types array. This update clarifies that types is used to exclude certain object types from the result, which is the opposite of how it's used in getTreeChilds(). The improved description should make the method easier to understand and use correctly in integrations.