There is a explanation about the behaviour of PluginToolbarButtons on the documentation page which also briefly explains how to use its members but otherwise, unlike what’s typically expected of a documentation page, its members are blank and so are the pages they lead to.
The following members pages are missing any documentation whatsoever and do not have a brief under their boxes on the PluginToolbarButton page:
Housekeeping: this is only partically resolved. Icon and SetActive are still undocumented. The rest of PluginToolbarButton’s members are documented (on the page itself in Creator Documentation as opposed to separate pages from the Developer Hub when this request was written).