[enekoreto]

Follow-up to the non-blocking docs suggestion in #934: say in the serde reference that the marker key is reserved.

Changes

• docs/reference/serde.rst: burr.core.serde.KEY is reserved, and a dict of your own that contains it is still saved and loaded back as is, wrapped as burr.dict.
• Same paragraph: don't use burr.dict as the key for your own deserializer, since register would replace Burr's handler (CanReader's note in the same PR).

How I tested this

• On main, a dict containing the key (also nested, and inside State) round-trips through serialize/deserialize.
• Registering a deserializer under burr.dict makes such a dict come back as whatever that function returns, so the warning is real.
• The docs build with no warnings, same as main, and pre-commit passes.

Notes

This was done with AI assistance (Claude Code), and checked by running the cases above.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.