docs: add histogram function

docs-cn/12-taos-sql/07-function.md

@@ -294,7 +294,7 @@ SELECT HISTOGRAM(field_name，bin_type, bin_description, normalized) FROM tb_nam
 
 **示例**：
 
-```sql
+```mysql
 taos> SELECT HISTOGRAM(voltage, "user_input", "[1,3,5,7]", 1) FROM meters;
          histogram(voltage, "user_input", "[1,3,5,7]", 1) |
      =======================================================

docs-en/12-taos-sql/07-function.md

@@ -275,11 +275,12 @@ SELECT HISTOGRAM(field_name，bin_type, bin_description, normalized) FROM tb_nam
 
 **Applicable table types**: table, STable
 
-**说明**：
+**Explanations**：
+
 1. bin_type: parameter to indicate the bucket type, valid inputs are: "user_input", "linear_bin", "log_bin"。
 2. bin_description: parameter to describe how to generate buckets，can be in the following JSON formats for each bin_type respectively: 
-       - "user_input": "[1, 3, 5, 7]"
-       User defined specified bin values.
+
+    - "user_input": "[1, 3, 5, 7]":  User specified bin values.
 
     - "linear_bin": "{"start": 0.0, "width": 5.0, "count": 5, "infinity": true}"
        "start" - bin starting point.
@@ -294,11 +295,12 @@ SELECT HISTOGRAM(field_name，bin_type, bin_description, normalized) FROM tb_nam
        "count" - number of bins generated.
        "infinity" - whether to add（-inf, inf）as start/end point in generated range of bins.
        The above "log_bin" descriptor generates a set of bins:[-inf, 1.0, 2.0, 4.0, 8.0, 16.0, +inf].
+
 3. normalized: setting to 1/0 to turn on/off result normalization.
 
-**示例**：
+**Example**：
 
-```sql
+```mysql
 taos> SELECT HISTOGRAM(voltage, "user_input", "[1,3,5,7]", 1) FROM meters;
      histogram(voltage, "user_input", "[1,3,5,7]", 1) |
      =======================================================