Remove references to _shard_doc

Opensearch does not currently appear to support `_shard_doc` as part of Point In Time search, so remove references to it from the documentation.

Further details:
- I don't see any reference to `_shard_doc` in the code on [Opensearch's](https://github.com/opensearch-project/OpenSearch) main branch at time of proposing the change.
- ElasticSearch added `_shard_doc` in [7.12](elastic/elasticsearch-net#5337) and it looks like it was not added as part of Opensearch's [Point In Time work](opensearch-project/OpenSearch#1147).

_search-plugins/point-in-time.md

@@ -55,8 +55,7 @@ GET /_search
     "keep_alive": "100m"
   },
   "sort": [ 
-    {"@timestamp": {"order": "asc", "format": "strict_date_optional_time_nanos"}},
-    {"_shard_doc": "desc"}
+    {"@timestamp": {"order": "asc", "format": "strict_date_optional_time_nanos"}}
   ]
 }
 ```
@@ -77,8 +76,7 @@ GET /_search
     "keep_alive": "100m"
   },
   "sort": [ 
-    {"@timestamp": {"order": "asc", "format": "strict_date_optional_time_nanos"}},
-    {"_shard_doc": "desc"}
+    {"@timestamp": {"order": "asc", "format": "strict_date_optional_time_nanos"}}
   ],
   "search_after": [  
     "2021-05-20T05:30:04.832Z"