On this page in the documentation, it describes node().

https://jenkins.io/doc/pipeline/steps/workflow-durable-task-step/#node-allocate-node

Some examples of using the operators have no quotes. Some have quotes around the individual components (eg. 'linux' || 'osx'). None have the full correct syntax (eg. 'linux || osx').

If you use the form with the quotes around the individual components, you get an error saying that node() does not allow a boolean argument. Of course, we are used to the error messaages occasionally having no useful information.

I suggest changing the documentation to NOT include the example with quotes around the components. (It says exactly this: For example, "osx (10.11)" || "Windows Server")

I also suggest putting something up at the top that shows a full example with an expression as well as a single agent's label.