Clarify low watermark documentation (#48112)

Today the docs say that the low watermark has no effect on any shards that have
never been allocated, but this is confusing. Here "shard" means "replication
group" not "shard copy" but this conflicts with the "never been allocated"
qualifier since one allocates shard copies and not replication groups.

This commit removes the misleading words. A newly-created replication group
remains newly-created until one of its copies is assigned, which might be quite
some time later, but it seems better to leave this implicit.

Author: renshuki

docs/reference/modules/cluster/disk_allocator.asciidoc

@@ -20,8 +20,7 @@ file or updated dynamically on a live cluster with the
     85% disk used. It can also be set to an absolute byte value (like `500mb`)
     to prevent Elasticsearch from allocating shards if less than the specified
     amount of space is available. This setting has no effect on the primary
-    shards of newly-created indices or, specifically, any shards that have
-    never previously been allocated.
+    shards of newly-created indices but will prevent their replicas from being allocated.
 
 `cluster.routing.allocation.disk.watermark.high`::