x/vuln: improve documentation

[zpavlinovic]

We should improve documentation on govulncheck:

• make it clear what are the main benefits of govulncheck, i.e., why should people use it
• explain how and why govulncheck can generate false positives and how to deal with them
• explain streaming JSON and OpenVex output as means of govulncheck integrations
• if applicable: how to filter vulnerabilities

[gabyhelp]

Related Issues and Documentation

• x/vuln: outdated govulncheck documentation #61152 (closed)
• x/vuln: feature request: govulncheck needs a (-f json) format option to output valid JSON for CI/CD tools to make decisions on the fly #60207 (closed)
• x/vuln: Missing output message when no vulnerabilities are found #61328 (closed)

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