Improve the schema explanation for the DynamoDB persistent storage option about launchdarkly-docs

[eli-darkly]

We do need to clarify these docs. The page you linked to does mention namespace and key rather than just key, but it's not immediately apparent that they are partition key & sort key (a.k.a. hash & range).

Showing a sample AWS command as you suggest might be helpful - however, I'm a little ambivalent about this because we definitely do not want to be misleadingly suggesting options that have nothing to do with the LaunchDarkly requirements, such as --billing-mode. I realize that the create-table command isn't valid unless you specify those options, but for things like the billing mode and the throughput limits, the developer really needs to understand what those are for and make decisions about them; we don't want anyone to just blindly copy our example.

[ember-stevens]

Docs updated on 4/1.