CLI spans: add section on the context propagation

[lmolkova]

With CLI span definition in #1588 we'll need to define context propagation from parent to child process following open-telemetry/oteps#258

Currently there is no spec definition and no propagators that implement it, but once spec is updated, the CLI spans should refer to it and recommend propagating the context when executing external CLI tools (and extracting context in them).

See #1588 (comment) for the context