docs: small rustdoc, guide and error message fixes from #709

[WeAreResilience]

Fixes #709

Applies the checklist items from #709:

• UnknownFields: rustdoc on the Extend and FromIterator impls.
• docs/guide.md: by-reference conversions now mention the numeric and bool wrappers, and that StringValue/BytesValue convert by value only.
• PoolError::NameTooLong: doc lists which names are checked (field and oneof names are not).
• symbol_conflicts.rs: renamed field_and_oneof_names_do_not_gain_a_length_limit to field_and_oneof_names_longer_than_max_symbol_len_link.
• PoolError::MessageSetWithFields: display text is now "MessageSet {message} declares a field; a MessageSet can have extensions only"; expected string in pool_e2e.rs updated; changelog fragment added.
• protoc version for edition 2024 corrected to v32 in CONTRIBUTING.md:8, buffa-test/build.rs, README.md, docs/guide.md and buffa-build/README.md.
• decode_from_str_with_element_memory_limit: first doc sentence says the budget is in bytes.

Not changed: CONTRIBUTING.md:88 and the conformance-local description in Taskfile.yml (protoc v30+ for the native conformance run), since the issue leaves those conditional on the reason being edition 2024.

Testing (toolchain 1.95.0):

• cargo test -p buffa-descriptor --all-features: all pass; messageset_declared_fields_are_rejected_transactionally fails with the old message and passes with the new one.
• cargo test -p buffa --all-features --lib unknown_fields: pass.
• cargo fmt --all -- --check, cargo clippy -p buffa -p buffa-descriptor --all-features --all-targets, RUSTDOCFLAGS="-D warnings" cargo doc -p buffa -p buffa-descriptor --all-features --no-deps: clean.

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[WeAreResilience]

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[iainmcgin]

[claude code] Pushed one commit (fe87875):

• Removed the Changed fragment. PoolError::MessageSetWithFields was added by #599 and is not in a release yet, so the release notes would describe a change to a text that users have not seen.
• buffa-test/src/tests/editions_2024.rs also said "protoc v30+"; it now says v32+. The issue did not list it.
• In the guide, the chrono and jiff sentence moved up beside the Timestamp and Duration sentence it refers to. With the wrapper sentences in between, "This" read as the wrappers. That placement came from the issue text.
• Five added lines are wrapped to the width of the lines around them.

[WeAreResilience]

Thanks — those fixes look right, especially dropping the unreleased Changed entry and moving the chrono/jiff sentence so This points at Timestamp/Duration again.