[DOCS] EQL: Document optional fields (#80150)
[DOCS] EQL: Document optional fields (#80150)

Adds new sections for optional fields and optional `by` fields. Also revises some existing content to define **join keys**.

Closes #79910

Relates to #79677 
GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch