docs: close the #719 sweep's last three leftovers (#719)

[JarryShaw]

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort)
• make test passes, and a test case covers the change (N/A: docstrings and one directive only; tests/project 275 passed, tests/test_docstring_contract.py 7 passed)
• Added a changelog entry: N/A, centralised in docs(changelog): shared 1.5.0 changelog — long-lived, merges last (#610, #616, #617, #618, #620) #657

What is the purpose of your pull request?

• fix — corrects a defect
• feat — adds a feature
• perf — changes performance, not behaviour
• refactor — changes neither behaviour nor performance
• test — tests only
• docs — documentation only
• ci — workflows or build tooling
• chore — anything else

Description of your pull request and other information

Part of #719. It closes the sweep's last three leftovers:

1. registry.rst gains .. autofunction:: for register_protocol_code. feat(protocols): opt-in code= registration with enum-type inference for Protocol/ProtocolBase #570 exported it in three __all__ lists but never documented it, so ext.rst's :func: reference resolved to nothing. Its docstring now links ProtocolBase rather than the undocumented __init_subclass__, which would have added a new warning.
2. utilities/logging.py: "pre-1.4" becomes "pre-1.5". The stderr handler is still attached at v1.4.1.post2, and Logging integration: make the logger a library citizen, and use it #384 first ships in v1.5.0a1.
3. toolkit/*.py: all eight module docstrings claimed the functions return "with a flag". They return data or None, and the pypcap/pcap_ct functions raise UnsupportedCall. The docstrings now match docs/source/pcapkit/toolkit/*.rst, and the PCAP-NG-NG typo is fixed.

Sphinx (-b dummy -n): 1290 warnings on main and 1289 on this branch, so 0 are new and the register_protocol_code one is gone.

File (AST with every bare-string Expr stripped, main vs branch)	Result
pcapkit/toolkit/{dpkt,pcap,pcap_ct,pcapng,pypcap,pypcapfile,pyshark,scapy}.py	identical (8/8)
pcapkit/foundation/registry/protocols.py, pcapkit/utilities/logging.py	identical (2/2)

[JarryShaw]

Cross-review verdict on 828d5064f: GOOD TO GO (ran on Sonnet; author Opus).

• register_protocol_code was missed, not hidden. It has a full docstring (protocols.py:301), it is in four __all__ lists, and it has been undocumented since feat(protocols): opt-in code= registration with enum-type inference for Protocol/ProtocolBase #570. The new autofunction sits directly after register_protocol. The rewritten docstring is accurate: ProtocolBase.__init_subclass__ (protocol.py:1027) takes code=.
• "pre-1.5" is correct. At v1.4.1.post2 the stderr handler is attached unconditionally (logging.py:55). git tag --contains 62a250abf puts Logging integration: make the logger a library citizen, and use it #384 first in v1.5.0a1.
• The toolkit docstrings now hold for all eight modules. The pcap, pcapng, dpkt and scapy adapters return data or None. The pypcap and pcap_ct adapters always raise UnsupportedCall. pypcapfile.ipv6_reassembly's raise is covered by the .. important:: block. Each leading sentence matches its .rst page. The "PCAP-NG Tools" title and its underline are both 13 characters.
• AST is identical for all 10 files, and every one has a real text diff.
• Tests: test_conventions_doc_claims 37 passed, test_documentation_claims 3 passed, test_docstring_contract 7 passed.

Nit (pre-existing, mirrored in 3rdparty.rst): pyshark.py says "reassembly and flow tracing adapters" but defines only tcp_traceflow.