docs: Clarify querybatch response mapping

[Haihan-Jiang]

Clarifies the /v1/querybatch response shape:

• each result maps to the query at the same index
• an empty result object means that query has no matched vulnerabilities
• returned vuln entries are id/modified summaries, not full OSV records
• clients should fetch full records with GET /v1/vulns/{id} and can use id + modified for cache freshness

Tests:

• git diff --check
• test -f docs/api/get-v1-vulns.md

Not run:

• bundle exec jekyll build; the local system Ruby is 2.6 and the docs lockfile requires Bundler 2.4.20.

Fixes #466.

[G-Rath]

Please read and follow our contributing guide before opening pull requests