Document match_description option for custom completions

[kiil]

Custom completers can now opt into matching the user's prefix against suggestion descriptions in addition to values, by setting match_description: true in the returned options record. The inserted completion is still the suggestion's value.

This PR documents the new option in book/custom_completions.md:

• Adds match_description to the list of completion options.
• Adds a "Matching against descriptions" example showing how a value (an email address) can be completed by matching against its description (a person's name).

🤖 Generated with Claude Code

[kiil]

The failing check spelling error is not in my patch.

The spelling error is in another part of the book:

https://www.nushell.sh/book/line_editor.html#abbreviations

[fdncred]

Thanks