[Docs] improved description for fs.total.available_in_bytes (#26657)

docs/reference/cluster/nodes-stats.asciidoc

@@ -96,7 +96,9 @@ information that concern the file system:
   Total number of unallocated bytes in all file stores
 
 `fs.total.available_in_bytes`::
-  Total number of bytes available to this Java virtual machine on all file stores
+  Total number of bytes available to this Java virtual machine on all file stores.
+  Depending on OS or process level restrictions, this might appear less than `fs.total.free_in_bytes`.
+  This is the actual amount of free disk space the Elasticsearch node can utilise.
 
 `fs.data`::
   List of all file stores
@@ -360,4 +362,4 @@ The `ingest` flag can be set to retrieve statistics that concern ingest:
 `ingest.total.failed`::
     The total number ingest preprocessing operations failed during the lifetime of this node
 
-On top of these overall ingest statistics, these statistics are also provided on a per pipeline basis.
+On top of these overall ingest statistics, these statistics are also provided on a per pipeline basis.