In that section, did you happen to read this sentence? 

"In addition, major-mode can be a list of several major modes specified in one of the above forms – this means a running instance of the associated server is responsible for files of multiple major modes or languages in the project. "

If so, how do you comprehend or interpret it? Maybe it could be clarified. Regardless, the default value of the eglot-server-programs variable that ships with Emacs has several examples that make use of this feature, so it may be worth studying those.

João