Polish WebAuthn Authentication Builder

Issue gh-17861

Author: jzheaux

webauthn/src/main/java/org/springframework/security/web/webauthn/authentication/WebAuthnAuthentication.java

@@ -81,8 +81,7 @@ public Builder<?> toBuilder() {
 	}
 
 	/**
-	 * A builder preserving the concrete {@link Authentication} type
-	 *
+	 * A builder of {@link WebAuthnAuthentication} instances
 	 * @since 7.0
 	 */
 	public static final class Builder<B extends Builder<B>> extends AbstractAuthenticationBuilder<B> {
@@ -94,6 +93,12 @@ private Builder(WebAuthnAuthentication token) {
 			this.principal = token.principal;
 		}
 
+		/**
+		 * Use this principal.
+		 * It must be of type {@link PublicKeyCredentialUserEntity}
+		 * @param principal the principal to use
+		 * @return the {@link Builder} for further configurations
+		 */
 		@Override
 		public B principal(@Nullable Object principal) {
 			Assert.isInstanceOf(PublicKeyCredentialUserEntity.class, principal,