[MINOR][DOCS] Document minimum executor memory requirement

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

This PR adds documentation for the minimum allowable executor memory value (450M) to prevent configuration errors especially when using offHeap memory configurations.

### Why are the changes needed?

Currently, Spark enforces a minimum executor memory of 450M as reserved memory to prevent OOMs, through validation logic in the below snippets
https://github.com/apache/spark/blob/1a802e36ed7698b7108ea7c0000efda1e6d5f949/core/src/main/scala/org/apache/spark/memory/UnifiedMemoryManager.scala#L264

https://github.com/apache/spark/blob/1a802e36ed7698b7108ea7c0000efda1e6d5f949/core/src/main/scala/org/apache/spark/memory/UnifiedMemoryManager.scala#L466

However, this constraint is not documented in the configuration guide or in the code-level configuration definition. The lack of documentation makes it hard for users to configure off-heap memory (spark.memory.offHeap.size) along with executor memory, leading to validation failures.

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

Only Doc changes

### How was this patch tested?

CI

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

No

Closes #53017 from pratham76/fix-docs.

Authored-by: Pratham Manja <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: Pratham Manja

core/src/main/scala/org/apache/spark/internal/config/package.scala

@@ -407,7 +407,8 @@ package object config {
     .createWithDefault(1)
 
   private[spark] val EXECUTOR_MEMORY = ConfigBuilder(SparkLauncher.EXECUTOR_MEMORY)
-    .doc("Amount of memory to use per executor process, in MiB unless otherwise specified.")
+    .doc("Amount of memory to use per executor process, in MiB unless otherwise specified, " +
+      "and with minimum value 450m")
     .version("0.7.0")
     .bytesConf(ByteUnit.MiB)
     .createWithDefaultString("1g")

docs/configuration.md

@@ -282,7 +282,8 @@ of the most common options to set are:
   <td>1g</td>
   <td>
     Amount of memory to use per executor process, in the same format as JVM memory strings with
-    a size unit suffix ("k", "m", "g" or "t") (e.g. <code>512m</code>, <code>2g</code>).
+    a size unit suffix ("k", "m", "g" or "t") (e.g. <code>512m</code>, <code>2g</code>),
+    and with minimum value <code>450m</code>.
   </td>
   <td>0.7.0</td>
 </tr>