[DOCS] Makes ELSER mapping requirements clearer (#96854)

Makes ELSER mapping requirements clearer.

Author: szabosteve

docs/reference/search/search-your-data/semantic-search-elser.asciidoc

@@ -38,6 +38,12 @@ that the model created based on your text - must be created.  The destination
 index must have a field with the <<rank-features, `rank_features`>> field type 
 to index the ELSER output.
 
+NOTE: ELSER output must be ingested into a field with the `rank_features` field 
+type. Otherwise, {es} interprets the token-weight pairs as a massive amount of 
+fields in a document. If you get an error similar to this `"Limit of total fields [1000] has been exceeded while adding
+new fields"` then the ELSER output field is not mapped properly and it has a 
+field type different than `rank_features`.
+
 [source,console]
 ----
 PUT my-index
@@ -47,16 +53,18 @@ PUT my-index
       "ml.tokens": {
         "type": "rank_features" <1>
       },
-      "text_field": {
-        "type": "text" <2>
+      "text": { <2>
+        "type": "text" <3>
       }
     }
   }
 }
 ----
 // TEST[skip:TBD]
 <1> The field that contains the prediction is a `rank_features` field.
-<2> The text field from which to create the sparse vector representation.
+<2> The name of the field from which to create the sparse vector representation. 
+In this example, the name of the field is `text`.
+<3> The field type which is text in this example.
 
 
 [discrete]
@@ -76,11 +84,11 @@ PUT _ingest/pipeline/elser-v1-test
       "inference": {
         "model_id": ".elser_model_1",
         "target_field": "ml",
-        "field_map": {
+        "field_map": { <1>
           "text": "text_field"
         },
         "inference_config": {
-          "text_expansion": { <1>
+          "text_expansion": { <2>
             "results_field": "tokens"
           }
         }
@@ -90,7 +98,10 @@ PUT _ingest/pipeline/elser-v1-test
 }
 ----
 // TEST[skip:TBD]
-<1> The `text_expansion` inference type needs to be used in the {infer} ingest 
+<1> The `field_map` object maps the input document field name (which is `text` 
+in this example) to the name of the field that the model expects (which is 
+always `text_field`).
+<2> The `text_expansion` inference type needs to be used in the {infer} ingest 
 processor.