MINOR: Add a description document for batchLength (#20140)

Add documentation for Batch Format to explain the meaning of
batchLength.

This is the preview image after the change:


![image](https://github.com/user-attachments/assets/85023c48-64e6-4a33-898f-df84f6864e58)

Reviewers: Ken Huang <[email protected]>, Jhen-Yung Hsu
<[email protected]>, Chia-Ping Tsai <[email protected]>

Author: gongxuanzhang

docs/implementation.html

@@ -59,6 +59,9 @@ <h4 class="anchor-heading"><a id="recordbatch" class="anchor-link"></a><a href="
 records: [Record]</code></pre>
     <p> Note that when compression is enabled, the compressed record data is serialized directly following the count of the number of records. </p>
 
+    <p>batchLength represents the number of bytes from the current position (immediately after the batchLength field) to the end of the batch. 
+    In other words, the total size of a record batch on disk is batchLength + 12 bytes, which includes the 8-byte baseOffset and the 4-byte batchLength field itself.</p>
+    
     <p>The CRC covers the data from the attributes to the end of the batch (i.e. all the bytes that follow the CRC). It is located after the magic byte, which
     means that clients must parse the magic byte before deciding how to interpret the bytes between the batch length and the magic byte. The partition leader
     epoch field is not included in the CRC computation to avoid the need to recompute the CRC when this field is assigned for every batch that is received by