Adding more master_is_stable details (#87977)

Adding information about new master_is_stable details fields

docs/reference/health/health.asciidoc

@@ -290,7 +290,8 @@ details have contents and a structure that is unique to each indicator.
     (Optional, array) A list of nodes that have been elected or replaced as master in a recent
     time window. This field is present if the master
     is changing rapidly enough to cause problems, and also present as additional information
-    when the indicator is `green`.
+    when the indicator is `green`. This array includes only elected masters, and does _not_
+    include empty entries for periods when there was no elected master.
 +
 .Properties of `recent_masters`
 [%collapsible%open]
@@ -303,17 +304,32 @@ details have contents and a structure that is unique to each indicator.
 ====
 
 `exception_fetching_history`::
-    (Optional, object) Master history is requested from the most recently elected master node for diagnosis purposes.
-    If fetching this remote history fails, the exception information is returned in this detail field.
+    (Optional, object) If the node being queried sees that the elected master has stepped down
+    repeatedly, the master history is requested from the most recently elected master node for
+    diagnosis purposes. If fetching this remote history fails, the exception information is
+    returned in this detail field.
 +
 .Properties of `exception_fetching_history`
 [%collapsible%open]
 ====
 `message`::
-(string) The exception message for the failed history fetch operation.
+    (string) The exception message for the failed history fetch operation.
 
 `stack_trace`::
-(string) The stack trace for the failed history fetch operation.
+    (string) The stack trace for the failed history fetch operation.
+====
+
+`cluster_formation`::
+    (Optional, object) If there has been no elected master node recently, the node being queried attempts to
+    gather information about why the cluster has been unable to form, or why the node being queried has been
+    unable to join the cluster if it has formed.
++
+.Properties of `cluster_formation`
+[%collapsible%open]
+====
+`description`::
+    (string) A detailed description explaining what went wrong with cluster formation, or why this node was
+    unable to join the cluster if it has formed.
 ====
 
 [[health-api-response-details-shards-availability]]