Update eligibility-signposting-api.yaml to v1.0.8-alpha

[jhadfield-NHS]

Adding draft description and refining some formatting and fixing quotations

Description

Added main bulk of description text, some formatting and quoting refinement

Context

APIM requires extensive documentation to be included in OAS specification

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.