Update search-indexer-field-mappings.md

Addressing all comments.

Author: shuyangmsft

articles/search/search-indexer-field-mappings.md

@@ -119,7 +119,7 @@ Performs *URL-safe* Base64 encoding of the input string. Assumes that the input
 
 #### Example - document key lookup
 
-Only URL-safe characters can appear in an Azure Cognitive Search document key (because customers must be able to address the document using the [Lookup API](https://docs.microsoft.com/rest/api/searchservice/lookup-document) ). If the source field for your key contains URL-unsafe characters, you can use the `base64Encode` function to convert it at indexing time.
+Only URL-safe characters can appear in an Azure Cognitive Search document key (because customers must be able to address the document using the [Lookup API](https://docs.microsoft.com/rest/api/searchservice/lookup-document) ). If the source field for your key contains URL-unsafe characters, you can use the `base64Encode` function to convert it at indexing time. However, a document key(both before and after convertion) can not be longer than 1024 characters.
 
 When you retrieve the encoded key at search time, you can then use the `base64Decode` function to get the original key value, and use that to retrieve the source document.
 
@@ -292,6 +292,8 @@ When you retrieve the encoded key at search time, you can then use the `urlDecod
   }]
  ```
 
+ <a name="fixedLengthEncodeFunction"></a>
+ 
  ### fixedLengthEncode function
 
  This function converts a string of any length to a fixed length string.
@@ -305,7 +307,7 @@ When facing errors complaining about document key being longer than 1024 charact
 "fieldMappings" : [
   {
     "sourceFieldName" : "metadata_storage_path",
-    "targetFieldName" : <your key field>,
+    "targetFieldName" : "your key field",
     "mappingFunction" : {
       "name" : "fixedLengthEncode"
     }