docs(core): document protobuf allocation behavior

[ddurst-nvidia]

Summary

• Document protobuf resource usage at the canonical outer decoder.
• Document caller-side limits for direct decoding through yaml_sigil_core::pb and link the configurable Buffa message-size bound.
• Forward allocation and input-limit guidance through the public core, transcription, signing transcoding, and verification entry points.
• Keep generated protobuf source unchanged; documentation remains at hand-written API boundaries.
• Clarify payload copying on successful verification.

Validation

• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features --package yaml-sigil-core --package yaml-sigil-transcription --package yaml-sigil-signing --package yaml-sigil-verification
• cargo xtask ci
• git diff --check

Impact

This is API documentation only. It has no compatibility, dependency, licensing, or release-version impact.

[ddurst-nvidia]

/ok to test 47841b6