Update docblocks' return types in client.lua

[steveoliver]

Summary

I found it confusing that the kong.client documentation said kong.client.get_credential() returns a string, but the @treturn and example usage indicate that in fact a table is returned. While I was in there, I saw other places where return values seem to have been mis-documented the other way around - where @treturn was indicated, but the return values were actually string.

I am not sure of all of the table items returned from get_credential(), but from what I can tell it is just id, consumer_id, and username. I need someone to confirm if that is all.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - (N/A - this PR handles the auto-generated PDK docs)
• Somebody besides @steveoliver confirms the items returned from ngx.ctx.authenticated_credential.

Issue reference

Fix #[issue number]

[steveoliver]

Anything else needed from me in order to move this forward?

[oowl]

LGTM