docs: document the reconnect_delay_ms producer option

[zmstone]

Make the existing reconnect_delay_ms producer option a first-class, documented option. No behavior change.

Background

reconnect_delay_ms is already accepted and honored by wolff_producer:

• Default 2000 ms (via use_defaults).
• Applied in do_ensure_delayed_reconnect/2 as reconnect_delay_ms + rand:uniform(1000) (1..1000 ms jitter, so partition workers do not all reconnect at the same instant).
• The first reconnect attempt right after a disconnect uses no delay (immediate); the delay applies to subsequent attempts.

But it was missing from the config_key() / config_in() / config_state() type specs and the producer option doc comment, so it was easy to miss and dialyzer did not recognise it as a valid producer config key.

Change

• Add reconnect_delay_ms to config_key(), config_in(), and config_state().
• Document it in the producer options doc comment (default, jitter, first-attempt-immediate behavior).
• Changelog note (unreleased 4.2.0).

Docs/types only — no runtime code path is touched.