docs(job): clarify JSON-serialization caveat for job.data (#2539)

[maruthang]

Port Impact Checklist

• Python – does this change need to be ported or documented in the Python library?
• Elixir – does this change need to be ported or documented in the Elixir library?
• PHP – does this change need to be ported or documented in the PHP library?

Why

The reporter in #2539 expected job.data.someMethod() to work after passing a class instance into the queue, but methods were missing. The current JSDoc and docs do not clearly warn that job.data is JSON-serialized through Redis on the way in and JSON-parsed on the way out, which strips prototypes/methods. The underlying behavior is correct and unchanged — this PR is a documentation-only clarification at the canonical user-facing surfaces.

Refs #2539

How

JSDoc edits on the four entry points where users encounter data:

• Job.data field — note that data is JSON-serialized when persisted, so only JSON-safe values survive a round trip; class instances lose their prototype/methods.
• Job.create — data param JSDoc states the same caveat.
• Queue.add — data param JSDoc states the same caveat.
• Queue.addBulk — JSDoc states that each job's data is serialized in the same way.

Plus a new section in docs/gitbook/guide/jobs/job-data.md:

• "Data is serialized as JSON" — explains the round trip explicitly and shows the recommended workaround pattern: producer sends a plain object, processor re-instantiates the class. Mentions toJSON() as an alternative when a custom serialized shape is desired.

No code changes, no test changes.

Additional Notes (Optional)

• Documentation-only PR. No runtime behavior changes.
• Verified runtime behavior in src/classes/job.ts (asJSON() / fromJSON() use plain JSON.stringify / JSON.parse with no toJSON revival hook on the parse side) so the docs match what the code actually does.
• The python/elixir/php ports likely have the same caveat in their respective docs and may warrant follow-up PRs in those directories. Out of scope for this Node-focused PR.

[manast]

remove dashes so that the text does not look so obviously written by an LLM.