doc processing doesn't handle glossary refs

[devoncarew]

See https://github.com/mdn/content/blob/main/files/en-us/web/api/selection/setbaseandextent/index.md?plain=1

...
The anchor and focus nodes can be located in a {{glossary("shadow tree")}}, if supported by the browser.
...

We should either emit a correct reference, or perhaps at least emit regular (unlinked) text. Currently we omit the entire construct (i.e., ... can be located in a , if supported ...).