Clarified dumpdata behavior for custom serializers. Refs #36608.

[irshadirshu0722]

Trac ticket number

ticket-36608

Branch description

Clarifies the documentation for custom serialization formats to explain that serializers are marked as internal by default and must set internal_use_only = False in order to be usable with management commands such as dumpdata. This addresses confusion reported in ticket #36608.

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Checklist

• This PR follows the contribution guidelines.
• This PR does not disclose a security vulnerability (see vulnerability reporting).
• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.

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