GH-3142: Clarify republishToDlq default behaviour in docs

The previous description opened with "By default, messages that fail after
retries are exhausted are rejected", which implies the default is false
(reject). The actual default is true (republish with extra headers).

Rewrite the description to lead with the true default behaviour and
describe the false case explicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>