[SPARK-50692][SQL][FOLLOWUP] Add comments for LPAD and RPAD

### What changes were proposed in this pull request?
This PR propose to add comments for `LPAD` and `RPAD`.

### Why are the changes needed?
#49325 had added pushdown support for `LPAD` and `RPAD`. But forget the comments.

### Does this PR introduce _any_ user-facing change?
'No'.
New feature.

### How was this patch tested?
GA.

### Was this patch authored or co-authored using generative AI tooling?
'No'.

Closes #49957 from beliefer/SPARK-50692_followup.

Authored-by: beliefer <[email protected]>
Signed-off-by: beliefer <[email protected]>

Author: beliefer

sql/catalyst/src/main/java/org/apache/spark/sql/connector/expressions/GeneralScalarExpression.java

@@ -416,6 +416,18 @@
  *    <li>Since version: 3.4.0</li>
  *   </ul>
  *  </li>
+ *  <li>Name: <code>LPAD</code>
+ *   <ul>
+ *    <li>SQL semantic: <code>LPAD(str, len[, pad])</code></li>
+ *    <li>Since version: 4.0.0</li>
+ *   </ul>
+ *  </li>
+ *  <li>Name: <code>RPAD</code>
+ *   <ul>
+ *    <li>SQL semantic: <code>RPAD(str, len[, pad])</code></li>
+ *    <li>Since version: 4.0.0</li>
+ *   </ul>
+ *  </li>
  * </ol>
  * Note: SQL semantic conforms ANSI standard, so some expressions are not supported when ANSI off,
  * including: add, subtract, multiply, divide, remainder, pmod.