Add documentation to SignatureVerification class

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/SignatureVerification.java

@@ -21,38 +21,83 @@ import org.bouncycastle.openpgp.PGPSignature;
 import org.pgpainless.exception.SignatureValidationException;
 import org.pgpainless.key.SubkeyIdentifier;
 
+/**
+ * Tuple of a signature and an identifier of its corresponding verification key.
+ * Semantic meaning of the signature verification (success, failure) is merely given by context.
+ * E.g. {@link OpenPgpMetadata#getVerifiedInbandSignatures()} contains verified verifications,
+ * while the class {@link Failure} contains failed verifications.
+ */
 public class SignatureVerification {
 
     private final PGPSignature signature;
     private final SubkeyIdentifier signingKey;
 
+    /**
+     * Construct a verification tuple.
+     *
+     * @param signature PGPSignature object
+     * @param signingKey identifier of the signing key
+     */
     public SignatureVerification(PGPSignature signature, @Nullable SubkeyIdentifier signingKey) {
         this.signature = signature;
         this.signingKey = signingKey;
     }
 
+    /**
+     * Return the {@link PGPSignature}.
+     *
+     * @return signature
+     */
     public PGPSignature getSignature() {
         return signature;
     }
 
+    /**
+     * Return a {@link SubkeyIdentifier} of the (sub-) key that is used for signature verification.
+     * Note, that this method might return null, e.g. in case of a {@link Failure} due to missing verification key.
+     *
+     * @return verification key identifier
+     */
     @Nullable
     public SubkeyIdentifier getSigningKey() {
         return signingKey;
     }
 
+    /**
+     * Tuple object of a {@link SignatureVerification} and the corresponding {@link SignatureValidationException}
+     * that caused the verification to fail.
+     */
     public static class Failure {
+
         private final SignatureVerification signatureVerification;
         private final SignatureValidationException validationException;
 
+        /**
+         * Construct a signature verification failure object.
+         *
+         * @param verification verification
+         * @param validationException exception that caused the verification to fail
+         */
         public Failure(SignatureVerification verification, SignatureValidationException validationException) {
             this.signatureVerification = verification;
             this.validationException = validationException;
         }
 
+        /**
+         * Return the verification (tuple of {@link PGPSignature} and corresponding {@link SubkeyIdentifier}
+         * of the signing/verification key.
+         *
+         * @return verification
+         */
         public SignatureVerification getSignatureVerification() {
             return signatureVerification;
         }
 
+        /**
+         * Return the {@link SignatureValidationException} that caused the verification to fail.
+         *
+         * @return exception
+         */
         public SignatureValidationException getValidationException() {
             return validationException;
         }