Backfill tests for RDoc::Stats#undoc_params

Part of #321
Ignore splat when checking for documented params

Previously we did not account for the param sigils in the arguments which
caused the param to be marked as undocumented.

Now we strip the splat sigil so the param gets correctly marked as
documented.

Part of #321.
Ignore keyword param when checking documentation

See @9f79384

Part of #321
Backfill block param documentation check test

Part of #321
Add #321 to History

Fixes #321