### Description

**Description**

We have been conducting user research on the copy and other aspects of the health API and have a few concrete copy suggestions from the user interviews. 

**StableMasterHealthIndicator**

1. We have user action `UNSTABLE_MASTER_DEPLOYMENT_MANAGEMENT_IMPACT` ([line](https://github.com/elastic/elasticsearch/blob/master/server/src/main/java/org/elasticsearch/cluster/coordination/StableMasterHealthIndicatorService.java#L62). I propose

_Before_: "Scheduled tasks such as Watcher, ILM, and SLM will not work. The _cat APIs will not work." 

_After_:  "Scheduled tasks such as Watcher, Index Lifecycle Management and Snapshot Lifecycle Management will not ...."  instead of using the abbreviations. Some users might not know the terms but at least this gives them more to search on and learn from. 

2. The `UNSTABLE_MASTER_BACKUP_IMPACT` ([line](https://github.com/elastic/elasticsearch/blob/100f5acd345dd13d8c8d17b93392d4b217bc50a2/server/src/main/java/org/elasticsearch/cluster/coordination/StableMasterHealthIndicatorService.java#L64)), I propose making the impact even clearer in laymen terms. 

_Before_: "Snapshot and restore will not work. Searchable snapshots cannot be mounted."
_After_: "Snapshot and restore will not work, your data will not be backed up. Searchable snapshots cannot be mounted, search on backup data will not return results."

I propose we do the same for the messages in the `SlmHealthIndicatorService`, and use the unabbreviated messages where shown to users. 
server/src/main/java/org/elasticsearch/cluster/routing/allocation/ShardsAvailabilityHealthIndicatorService.java
public static final String DIAGNOSE_SHARDS_ACTION_GUIDE = "http://ela.st/diagnose-shards";
    public static final Diagnosis.Definition ACTION_CHECK_ALLOCATION_EXPLAIN_API = new Diagnosis.Definition(
        NAME,
        "explain_allocations",
        "Elasticsearch isn't allowed to allocate some shards from these indices to any of the nodes in the cluster.",
        "Diagnose the issue by calling the allocation explain API for an index [GET _cluster/allocation/explain]. Choose a node to which "
            + "you expect a shard to be allocated, find this node in the node-by-node explanation, and address the reasons which prevent "
            + "Elasticsearch from allocating the shard.",
        DIAGNOSE_SHARDS_ACTION_GUIDE
    );
    public static final String FIX_DELAYED_SHARDS_GUIDE = "http://ela.st/fix-delayed-shard-allocation";
    public static final Diagnosis.Definition DIAGNOSIS_WAIT_FOR_OR_FIX_DELAYED_SHARDS = new Diagnosis.Definition(
        NAME,
        "delayed_shard_allocations",
        "Elasticsearch is not allocating some shards because they are marked for delayed allocation. Shards that have become "
            + "unavailable are usually marked for delayed allocation because it is more efficient to wait and see if the shards return "
            + "on their own than to recover the shard immediately.",
        "Elasticsearch will reallocate the shards when the delay has elapsed. No action is required by the user.",
        FIX_DELAYED_SHARDS_GUIDE
    );
    public static final String ENABLE_INDEX_ALLOCATION_GUIDE = "http://ela.st/fix-index-allocation";
    public static final Diagnosis.Definition ACTION_ENABLE_INDEX_ROUTING_ALLOCATION = new Diagnosis.Definition(
        NAME,
        "enable_index_allocations",
        "Elasticsearch isn't allowed to allocate some shards from these indices because allocation for those shards has been disabled at "
            + "the index level.",
        "Check that the ["
            + INDEX_ROUTING_ALLOCATION_ENABLE_SETTING.getKey()
            + "] index settings are set to ["
            + EnableAllocationDecider.Allocation.ALL.toString().toLowerCase(Locale.getDefault())