Motivation
As a Pulsar newbie, the role of "partition key" and "ordering key" aren't entirely clear from the Doxygen comments. These names are not used in the public Pulsar documentation that I saw, so expanding the comments felt useful to me and may help other users.

Modifications
Expanded doc strings for MessageBuilder.setPartitionKey and setOrderingKey

Verifying this change
This change is a trivial rework / code cleanup without any test coverage.

Documentation
This change improves the C++ Doxygen output.