Add clarification about attribute types to Telemetry RFC (#12407)

As suggested in
#12217 (comment)

Co-authored-by: Pablo Baeyens <[email protected]>

Author: djaglowski

docs/rfcs/component-universal-telemetry.md

@@ -27,7 +27,7 @@ be automatically captured for each kind of pipeline component.
 
 ## Attributes
 
-All signals should use the following attributes:
+Spans, data points, and log records should carry the following attributes:
 
 ### Receivers