[SPARK-29542][FOLLOW-UP] Keep the description of spark.sql.files.* in tuning guide be consistent with that in SQLConf

### What changes were proposed in this pull request?
This pr is a follow up of #26200.

In this PR, I modify the description of spark.sql.files.* in sql-performance-tuning.md to keep consistent with that in SQLConf.

### Why are the changes needed?

To keep consistent with the description in SQLConf.

### Does this PR introduce any user-facing change?
No.

### How was this patch tested?
Existed UT.

Closes #27545 from turboFei/SPARK-29542-follow-up.

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

Author: turboFei

docs/sql-performance-tuning.md

@@ -67,6 +67,7 @@ that these options will be deprecated in future release as more optimizations ar
     <td>134217728 (128 MB)</td>
     <td>
       The maximum number of bytes to pack into a single partition when reading files.
+      This configuration is effective only when using file-based sources such as Parquet, JSON and ORC.
     </td>
   </tr>
   <tr>
@@ -76,7 +77,8 @@ that these options will be deprecated in future release as more optimizations ar
       The estimated cost to open a file, measured by the number of bytes could be scanned in the same
       time. This is used when putting multiple files into a partition. It is better to over-estimated,
       then the partitions with small files will be faster than partitions with bigger files (which is
-      scheduled first).
+      scheduled first). This configuration is effective only when using file-based sources such as Parquet,
+      JSON and ORC.
     </td>
   </tr>
   <tr>