[DOCS] Add multi-field def to glossary (elastic#1700)

Author: jrodewig

docs/en/glossary/glossary.asciidoc

@@ -814,6 +814,12 @@ A network endpoint which is monitored to track the performance and availability
 applications and services.
 //Source: Observability
 
+[[glossary-multi-field]] multi-field::
++
+--
+include::{es-repo-dir}/glossary.asciidoc[tag=multi-field-def]
+--
+
 [discrete]
 [[n-glos]]
 === N