Document clearly that Supplier should operate on singly-valued keys

[krnowak]

This is mostly a note to self, so I can add the documentation during my work on context propagation. This is to resolve an issue raised in #441.

[krnowak]

So this issue conflicts with the following issues in the otel-spec (as we will likely need to rework the supplier interface for allowing multi-valued keys and rework propagator interface to have an iterator over keys instead of a function for getting a fixed list of the keys the propagator uses/understands):

open-telemetry/opentelemetry-specification#433
open-telemetry/opentelemetry-specification#443
open-telemetry/opentelemetry-specification#444