To change the document options while editing a symbol within a schematic library, go to Tools ► Document Options, which will open the Properties panel. The unit settings and the Comment/Designator visibility can then be changed under the 'General' section of the panel.
Here's a link to the relevant section of documentation on the Properties panel with more detailed information on the settings available in this panel: https://www.altium.com/documentation/altium-designer/interactiveproperties-pnl-propertiesproperties-ad#schematic-library-support