[repo-assist] docs: add 4.0.0-beta04 release notes entry

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Summary

Adds the 4.0.0-beta04 release notes entry to docs/RELEASE_NOTES.md documenting the improvements merged since beta03 (May 1, 2026):

• [Repo Assist] improve: surface schema description as XmlDoc on generated object and enum types (+5 tests, 348→353) #419 – improve: surface schema description as XmlDoc on generated object and enum types (+5 tests)
• [Repo Assist] improve: add (returns) XmlDoc tag for operation response descriptions (+4 tests, 353→357) #420 – improve: add <returns> XmlDoc tag for operation response descriptions (+4 tests)
• [Repo Assist] test: add direct unit tests for XmlDoc module (+32 tests, 357→389) #422 – test: add direct unit tests for XmlDoc module (+32 tests, 357→389 total)

Test Status

Documentation-only change — no code modified.

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