correct way of getting node heap size (#85045)

* correct way of getting node heap size

in [[shard-count-recommendation]], we explain that the number of shards should be at most 20 shards per GB of heap. 
but the command to get relevant heap size should be _cat/nodes?v=true&h=heap.max and not _cat/nodes?v=true&h=heap.current . The latter gives the current memory consumption, which is alway moving. Here we need to consider the max allocated heap size (-Xmx)

* Adds heap.max to valid columns

Co-authored-by: Adam Locke <adam.locke@elastic.co>

docs/reference/cat/nodes.asciidoc

@@ -48,6 +48,9 @@ Valid columns are:
 `heap.percent`, `hp`, `heapPercent`::
 (Default) Maximum configured heap, such as `7`.
 
+`heap.max`, `hm`, `heapMax`::
+(Default) Total heap, such as `4gb`.
+
 `ram.percent`, `rp`, `ramPercent`::
 (Default) Used total memory percentage, such as `47`.
 

docs/reference/how-to/size-your-shards.asciidoc

@@ -187,12 +187,12 @@ Some system indices for {enterprise-search-ref}/index.html[Enterprise Search]
 are nearly empty and rarely used. Due to their low overhead, you shouldn't
 count shards for these indices toward a node's shard limit.
 
-To check the current size of each node's heap, use the <<cat-nodes,cat nodes
+To check the configured size of each node's heap, use the <<cat-nodes,cat nodes
 API>>.
 
 [source,console]
 ----
-GET _cat/nodes?v=true&h=heap.current
+GET _cat/nodes?v=true&h=heap.max
 ----
 // TEST[setup:my_index]