docs: add json to full text search document

docs/src/format/table/index/scalar/fts.md

@@ -129,6 +129,66 @@ Token filters are applied in sequence after the base tokenizer:
 For stemming and stop word removal, the following languages are supported:
 Arabic, Danish, Dutch, English, Finnish, French, German, Greek, Hungarian, Italian, Norwegian, Portuguese, Romanian, Russian, Spanish, Swedish, Tamil, Turkish
 
+## Document Type
+Lance supports 2 kinds of documents: text and json. Different document types have different tokenization rules, and
+parse tokens in different format.
+
+### Text Type
+Text type includes text and list of text. Tokens are generated by base_tokenizer.
+
+The example below shows how text document is parsed into tokens. 
+```text
+Tom lives in San Francisco.
+```
+
+The tokens are below.
+```text
+Tom
+lives
+in
+San
+Francisco
+```
+
+### Json Type
+Json is a nested structure, lance breaks down json document into tokens in triplet format `path,type,value`. The valid
+types are: str, number, bool, null.
+
+In scenarios where the triplet value is a str, the text value will be further tokenized using the base_tokenizer,
+resulting in multiple triplet tokens.
+
+During querying, the Json Tokenizer uses the triplet format instead of the json format, which simplifies the query
+syntax.
+
+The example below shows how the json document is tokenized. Assume we have the following json document:
+```json
+{
+  "name": "Lance",
+  "legal.age": 30,
+  "address": {
+    "city": "San Francisco",
+    "zip:us": 94102
+  }
+}
+```
+
+After parsing, the document will be tokenized into the following tokens:
+```
+name,str,Lance
+legal.age,number,30
+address.city,str,San
+address.city,str,Francisco
+address.zip:us,number,94102
+```
+
+Then we do full text search in triplet format. To search for "San Francisco," we can search with one of the triplets
+below:
+```
+address.city:San Francisco
+address.city:San
+address.city:Francisco
+```
+
 ## Accelerated Queries
 
 Lance SDKs provide dedicated full text search APIs to leverage the FTS index capabilities.