GUACAMOLE-1949: Add Javadoc for getUserId method

extensions/guacamole-auth-nextcloud/src/main/java/org/apache/guacamole/auth/nextcloud/NextcloudJwtAuthenticationProvider.java

@@ -291,6 +291,21 @@ private boolean isUserAllowed(String uid) throws GuacamoleException {
         return confService.getAllowedUser().contains(uid);
     }
 
+    /**
+     * Decodes a Base64 encoded JSON payload and extracts the uid
+     *
+     * <p>This method takes a Base64 encoded string as input, decodes it to a JSON string,
+     * parses the JSON to extract the user ID from the "userdata" object.
+     *
+     * @param payload
+     *     The Base64 encoded JSON string containing user data.
+     *
+     * @return
+     *     The user ID extracted from the decoded JSON payload.
+     *
+     * @throws JsonProcessingException
+     *     If there is an error processing the JSON payload.
+     */
     private String getUserId(String payload) throws JsonProcessingException {
         byte[] decodedBytes = Base64.getDecoder().decode(payload);
         String decodedPayload = new String(decodedBytes, StandardCharsets.UTF_8);