Merge pull request #24906 from derekwaynecarr/memory-backed-size

Document SizeMemoryBackedVolumes feature for 1.20

content/en/docs/concepts/storage/volumes.md

@@ -303,6 +303,12 @@ While tmpfs is very fast, be aware that unlike disks, tmpfs is cleared on
 node reboot and any files you write count against your container's
 memory limit.
 
+{{< note >}}
+If the `SizeMemoryBackedVolumes` [feature gate](/docs/reference/command-line-tools-reference/feature-gates/) is enabled,
+you can specify a size for memory backed volumes.  If no size is specified, memory
+backed volumes are sized to 50% of the memory on a Linux host.
+{{< /note>}}
+
 #### emptyDir configuration example
 
 ```yaml