Doc improvements (#3611)
Doc improvements (#3611)

* Doc improvements

* function -> func to match Go syntax
* clarify that AttributeValue is actually a mutable cell

* Range() code examples shouldn't have a return value

* Fix return type in doc
open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector