docs(internal): document ProcessBuffer and continuation-state helper invariants

[lodyai]

Background

Several intricate private state-machine helpers carry no /// documentation,
contrary to the AGENTS.md guidance to comment the why (assumptions, edge
cases, trade-offs):

• ProcessBuffer methods in src/process.rs:63-161 — flush, push_verbatim,
  handle_fence_line, handle_table_line.
• Continuation-state helpers in src/wrap/continuation.rs —
  reopen_pending_span (:113), join_pending_continuation (:210),
  suppresses_join_space_after_nested_open_paren (:280),
  unclosed_parenthesis_depth (:292).

Problem

These functions encode non-obvious invariants (fence-width edge cases,
CommonMark soft-break semantics, parenthesis-depth tracking). Without doc
comments stating what each maintains and which edge case it exists for, a future
maintainer must reverse-engineer the intent from the control flow.

Objective

Add a short /// doc comment to each listed function (and method) stating:

• the invariant it maintains over the shared state, and
• the specific edge case or rule (e.g. CommonMark soft-break behaviour, fence
  width comparison) that justifies its existence.

Comments must follow the project's en-GB-oxendict spelling convention and
explain why, not restate what the code does.

Acceptance criteria

• Every function/method listed above has a /// doc comment.
• make lint/doc gates pass.

References

• AGENTS.md — "Comment why, not what."
• Requested by @leynos

[coderabbitai]

🔗 Related PRs

#275 - Preserve nested fenced blocks (#262) [merged]
#302 - Keep inflectional affixes attached to inline code spans during reflow (#300) [merged]
#308 - Keep hyphenated compounds with inline code spans atomic during wrap (#307) [merged]
#343 - Preserve fenced code block contents during wrapping (#329) [merged]
#356 - Keep escaped backtick code spans atomic (#354) [closed]

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