[SPARK-44718][FOLLOWUP][DOCS] Avoid using ConfigEntry in spark.sql.co…

…lumnVector.offheap.enabled's doc field

### What changes were proposed in this pull request?

Followup of #42394
```
   * - spark.sql.columnVector.offheap.enabled
     - When true, use OffHeapColumnVector in ColumnarBatch. Defaults to ConfigEntry(key=spark.memory.offHeap.enabled, defaultValue=false, doc=If true, Spark will attempt to use off-heap memory for certain operations. If off-heap memory use is enabled, then spark.memory.offHeap.size must be positive., public=true, version=1.6.0).
     - <value of spark.memory.offHeap.enabled>
     - 2.3.0
```

The doc field shall be interpolated by MEMORY_OFFHEAP_ENABLED.key instead of MEMORY_OFFHEAP_ENABLED. In this PR, we remove the doc redundant doc as it's also can be found in the `MEMORY_OFFHEAP_ENABLED.defaultValueString`
### Why are the changes needed?

docfix

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

manually debugging

### Was this patch authored or co-authored using generative AI tooling?

no

Closes #47165 from yaooqinn/minor2.

Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -535,8 +535,7 @@ object SQLConf {
   val COLUMN_VECTOR_OFFHEAP_ENABLED =
     buildConf("spark.sql.columnVector.offheap.enabled")
       .internal()
-      .doc("When true, use OffHeapColumnVector in ColumnarBatch. " +
-        s"Defaults to $MEMORY_OFFHEAP_ENABLED.")
+      .doc("When true, use OffHeapColumnVector in ColumnarBatch.")
       .version("2.3.0")
       .fallbackConf(MEMORY_OFFHEAP_ENABLED)