You can generate documentation for XML Schemas from Oxygen XML Author Eclipse plugin by using the main menu action. The settings dialog box has an Export settings option that can be used to export the settings to an XML configuration file. Once the settings are exported, you can use the schemaDocumentation.sh script (found in the scripts subfolder inside Oxygen's installation directory) to generate XML Schema documentation from the command line.
sh scripts/schemaDocumentation.sh schemaFile [-cfg:configFile] [-out:outputFile]
[-format:<value>] [-xsl:xslFile] [-split:<value>] [-openInBrowser:<value>]