docs(footnotes): add doc comments to parse_definition and is_definition_continuation

[lodyai]

Background

The footnotes parsing module exposes two pub(super) entry points without doc
comments:

• parse_definition (src/footnotes/parsing.rs:24)
• is_definition_continuation (src/footnotes/parsing.rs:36)

Problem

These are the module's parsing interface, consumed across the footnotes
submodules, yet a reader has no documented contract for their inputs, return
values, or the definition/continuation grammar they encode.

Objective

Add a concise /// doc comment to each, describing inputs and return value:

• parse_definition — what a footnote definition line is parsed into, and when
  it returns None.
• is_definition_continuation — what makes a line a continuation of a preceding
  definition.

Comments must use en-GB-oxendict spelling.

Acceptance criteria

• Both functions have /// doc comments stating inputs and outputs.
• make lint/doc gates pass.

References

• AGENTS.md — documentation conventions.
• Requested by @leynos

[coderabbitai]

🔗 Related PRs

leynos/vk#189 - Add cargo-binstall Linux release support [merged]
#280 - Keep GFM footnote references atomic (#277) [merged]
#355 - Preserve inline GFM footnote reference attachment (#353) [merged]

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