docs(shopping-list): document --pantry, --aisle and related flags

[dubadub]

Summary

Fleshes out the terse help text for cook shopping-list's --pantry, --aisle, --ignore-pantry and --ignore-references flags so they match the detailed, example-rich style of the other options.

Context

In #333 a user couldn't find/use --pantry. The flag itself was added in #340 (shipped in 0.30.0); the reporter was on 0.29.1. But the issue also flagged that the option was poorly documented in --help. Each of these flags previously had only a one-line entry — this expands them to explain behaviour and the config auto-discovery (./config/ then global config dir).

No behavioural change; documentation only.

Verification

• cargo build, cargo fmt, cargo clippy all clean
• cook shopping-list --help renders the expanded entries

Refs #333

[claude]

Review — docs(shopping-list): document --pantry, --aisle and related flags

Summary: Documentation-only PR; no behaviour changes. Overall this is solid and clearly addresses the discoverability gap raised in #333.

What's good

• All four descriptions are accurate and consistent with how the flags actually work (verified against src/shopping_list.rs).
• Adding value_hint = clap::ValueHint::FilePath on --aisle and --pantry is a nice bonus — it enables better shell-completion suggestions. Worth calling out in the PR description since it's a functional improvement, not just docs.
• The cross-reference from --pantry to --ignore-pantry ("Use --ignore-pantry to skip pantry subtraction entirely.") mirrors the style of the recipes positional arg — good consistency.
• cargo fmt / cargo clippy verification noted.

Minor suggestions

1. --ignore-references example syntax — The description says @./other.cook to illustrate recipe references. Worth double-checking this matches the actual accepted parser syntax exactly, so new users copying from --help don't paste something that silently fails.

2. Config dir path on macOS — Both --aisle and --pantry say "global config directory (~/.config/cook/ or the platform equivalent)". macOS uses ~/Library/Application Support/cook/. Spelling out both paths explicitly would be clearer than "platform equivalent" for macOS users reading --help.

3. --ignore-pantry wording — "skips loading the pantry file altogether" is slightly redundant with "includes every ingredient regardless of what's in the pantry". One of those sentences is enough.

These are all nits — the PR is ready to merge as-is.