[kimseongchan0914]

Fixes #1070

Summary

The JwtParser.parse Javadoc only mentioned ExpiredJwtException, while DefaultJwtParser also rejects tokens whose nbf is in the future. This PR documents the claims validation that actually happens during parsing:

• Adds a Claims Validation section to parse(CharSequence) describing:
  • exp → ExpiredJwtException, subject to the configured clock and clock skew
  • nbf → PrematureJwtException, subject to the configured clock and clock skew
  • require* claims → MissingClaimException / IncorrectClaimException
• Explicitly notes that iat is not validated against the current time. The issue mentions iat, but DefaultJwtParser only checks exp and nbf.
• Adds the missing @throws PrematureJwtException to parse(CharSequence), parse(CharSequence, JwtHandler) and parseClaimsJws(CharSequence), next to the existing @throws ExpiredJwtException tags.

This is a Javadoc-only change; behavior is unchanged.

Testing

• ./mvnw -pl api javadoc:javadoc builds without errors.

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