Clarify that Setter sets values only. (open-telemetry#976)

Author: carlosalberto

specification/context/api-propagators.md

@@ -144,8 +144,8 @@ the base [Inject](#inject) operation.
 
 Optional arguments:
 
-- A `Setter` invoked for each propagation key to add or remove. This is an additional
-  argument that languages are free to define to help inject data into the carrier.
+- A `Setter` to set a propagation key/value pair. Propagators MAY invoke it multiple times in order to set multiple pairs.
+  This is an additional argument that languages are free to define to help inject data into the carrier.
 
 #### Setter argument
 
@@ -263,7 +263,7 @@ Required arguments:
 
 - A `Context`.
 - The carrier that holds propagation fields.
-- The `Setter` invoked for each propagation key to add or remove.
+- The `Setter` to set a propagation key/value pair. Propagators MAY invoke it multiple times in order to set multiple pairs.
 
 ## Global Propagators