Update build-integration.asciidoc (#3757)

Small addition to the explanation of what each file in the data_stream/fields directory does. 
- agent.yml provides the fields the Elastic agent uses.
- fields.yml are additional fields the integration uses.

docs/en/integrations/build-integration.asciidoc

@@ -215,10 +215,10 @@ apache
 │   └───error
 │   └───status
 ----
-<1> ??
+<1> `agent.yml` fields the Elastic agent uses
 <2> `base-fields.yml` never changes and is required for all integrations
 <3> Defines the relevant ECS fields
-<4> Custom Apache access log fields ??
+<4> Custom Apache access log fields
 
 
 // Need more on mapping