Suggest capturing a heap dump to diagnose high heap (#96526)

The `high-jvm-memory-pressure.html` troubleshooting docs give some
suggestions, but vitally they omit the advice to capture a heap dump
which is what we really need users to do if they want to understand
their high heap usage. This commit adds a note to the docs to that
effect.

docs/reference/how-to/fix-common-cluster-issues.asciidoc

@@ -158,6 +158,8 @@ You can also use the <<cat-nodes,cat nodes API>> to get the current
 GET _cat/nodes?v=true&h=name,node*,heap*
 ----
 
+See <<high-jvm-memory-pressure>> for more details.
+
 To get the JVM memory usage for each circuit breaker, use the
 <<cluster-nodes-stats,node stats API>>.
 

docs/reference/troubleshooting/common-issues/high-jvm-memory-pressure.asciidoc

@@ -27,10 +27,17 @@ collection.
 [timestamp_short_interval_from_last][INFO ][o.e.m.j.JvmGcMonitorService] [node_id] [gc][number] overhead, spent [21s] collecting in the last [40s]
 ----
 
+**Capture a JVM heap dump**
+
+To determine the exact reason for the high JVM memory pressure, capture a heap
+dump of the JVM while its memory usage is high.
+
 [discrete]
 [[reduce-jvm-memory-pressure]]
 ==== Reduce JVM memory pressure
 
+This section contains some common suggestions for reducing JVM memory pressure.
+
 **Reduce your shard count**
 
 Every shard uses memory. In most cases, a small set of large shards uses fewer
@@ -92,4 +99,4 @@ between them.
 **Upgrade node memory**
 
 Heavy indexing and search loads can cause high JVM memory pressure. To better
-handle heavy workloads, upgrade your nodes to increase their memory capacity.
+handle heavy workloads, upgrade your nodes to increase their memory capacity.