[DOCS] Task management API experimental status issue

Add issue reference to documentation.

Relates elastic#51628

Author: henningandersen

docs/reference/cat/tasks.asciidoc

@@ -4,7 +4,7 @@
 <titleabbrev>cat task management</titleabbrev>
 ++++
 
-beta::[The cat task management API is new and should still be considered a beta feature. The API may change in ways that are not backwards compatible.]
+beta::["The cat task management API is new and should still be considered a beta feature. The API may change in ways that are not backwards compatible.",{issue}51628]
 
 Returns information about tasks currently executing in the cluster,
 similar to the <<tasks,task management>> API.

docs/reference/cluster/tasks.asciidoc

@@ -4,7 +4,7 @@
 <titleabbrev>Task management</titleabbrev>
 ++++
 
-beta[The task management API is new and should still be considered a beta feature. The API may change in ways that are not backwards compatible.]
+beta::["The task management API is new and should still be considered a beta feature. The API may change in ways that are not backwards compatible.",{issue}51628]
 
 Returns information about the tasks currently executing in the cluster.