docs(protocols): second-round pass over the protocol pages (#719)

[JarryShaw]

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort) — N/A, docs-only (Sphinx -n and tests/project below)
• make test passes, and a test case covers the change — tests/project passes; prose only
• 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

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• docs — documentation only

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Second-round #719 read of all 46 pages under docs/source/pcapkit/protocols/; 13 changed, 33 left as they were. Every fix is a claim the code contradicts:

• "contains X only" was false on ftp, rarp, arp, ngap, esp and protocol (checked against each module's __all__). protocol.rst also called Protocol the base of every family, but the built-in families subclass ProtocolBase.
• ipx: the field is ipx.chksum. mh: removed the todo saying CGA Parameters (type 12) still used the generic handler, since MH.__option__ routes it to cga_param.
• index: NGAP was missing from the class-hierarchy graph. application/index: the APPTYPE alias target was unresolved; it now points to pcapkit.const.reg.apptype.apptype.AppType.
• misc/index now names PCAPNG, and c_tag/s_tag now include id() in the identity members they add.

sphinx -b dummy -n on fresh output directories, counting warnings on these pages only: main 1, branch 0, none new.

[JarryShaw]

Cross-review verdict on 8e796370b: NEEDS CHANGES (ran on Sonnet; author Opus). There is one wording fix to make.

protocol.rst now says Protocol is the base class for user-defined protocols. That is half true. ext.rst:112-116 tells authors to subclass Link/Internet/Transport/Application, and to inherit Protocol directly only for a new protocol stack. Please say that instead.

Everything else checks out:

• Every name the pages now list matches its module's __all__: ftp, rarp, arp, ngap, and all 10 in esp.
• Probed: Protocol.__subclasses__() is [], HTTP resolves through Application to ProtocolBase, and type(Protocol) is ProtocolMeta.
• ipx uses chksum.
• The mh CGA_Parameters handler exists (mh.py:1217, :3288).
• NGAP subclasses Application.
• The APPTYPE target resolves, and C_Tag/S_Tag define id().
• No directive target changed.
• tests/project passes 379.

UNVERIFIED: no Sphinx build.

[JarryShaw]

Cross-review verdict on a34eea95b: GOOD TO GO (ran on Sonnet; author Opus, round 2)

The delta from 8e796370b touches only one paragraph in protocol.rst.

• "Inherit Protocol directly only for a new stack; a layer protocol subclasses Link/Internet/Transport/Application" matches ext.rst:98-114.
• The built-ins, including those four layer classes, are class X(ProtocolBase[...]), and type(ProtocolBase) is ProtocolMeta, as probed in round one.
• All four new :class: targets have autoclass entries, and :doc: /ext resolves.
• The round-one findings stand.

Not re-run for this prose-only delta: tests and Sphinx. The worker reports Sphinx -n as unchanged and tests/project at 379 passed.