add doc of json_extract_float64 and json_extract_string

[yangj1211]

What type of PR is this?

• Enhancement
• Displaying
• Typo
• Doc Request

Which issue(s) this PR fixes:

issue #588,#1134

What this PR does / why we need it:

[aronchanisme]

这里有个地方需要注意一下：

mysql> create table test_123(c1 json);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into test_123 values ('{"a1":10, "a2":20}');
Query OK, 1 row affected (0.01 sec)

mysql> select * from test_123;
+----------------------+
| c1                   |
+----------------------+
| {"a1": 10, "a2": 20} |
+----------------------+
1 row in set (0.00 sec)

mysql> select json_extract_float64(c1, '$.a1, $.a2') from test_123;
ERROR 20301 (HY000): invalid input: invalid json path '$.a1, $.a2'
mysql> select json_extract_float64(c1, '$.a1', '$.a2') from test_123;
+--------------------------------------+
| json_extract_float64(c1, $.a1, $.a2) |
+--------------------------------------+
|                                 NULL |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> select json_extract_float64(c1, '$.a1') from test_123;
+--------------------------------+
| json_extract_float64(c1, $.a1) |
+--------------------------------+
|                             10 |
+--------------------------------+
1 row in set (0.00 sec)

mysql> select json_extract(c1, '$.a1') from test_123;
+------------------------+
| json_extract(c1, $.a1) |
+------------------------+
| 10                     |
+------------------------+
1 row in set (0.00 sec)

mysql> select json_extract(c1, '$.a1', '$.a2') from test_123;
+------------------------------+
| json_extract(c1, $.a1, $.a2) |
+------------------------------+
| [10, 20]                     |
+------------------------------+
1 row in set (0.00 sec)

mysql> select json_extract_float64(c1, '$.a1', '$.a2') from test_123;
+--------------------------------------+
| json_extract_float64(c1, $.a1, $.a2) |
+--------------------------------------+
|                                 NULL |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> select json_extract_string(c1, '$.a1', '$.a2') from test_123;
+-------------------------------------+
| json_extract_string(c1, $.a1, $.a2) |
+-------------------------------------+
| NULL                                |
+-------------------------------------+
1 row in set (0.00 sec)

mysql> insert into test_123 values ('{"a1":"test", "a2":"test2"}');
Query OK, 1 row affected (0.00 sec)

mysql> select * from test_123;
+-------------------------------+
| c1                            |
+-------------------------------+
| {"a1": 10, "a2": 20}          |
| {"a1": "test", "a2": "test2"} |
+-------------------------------+
2 rows in set (0.00 sec)

mysql> select json_extract_string(c1, '$.a1', '$.a2') from test_123;
+-------------------------------------+
| json_extract_string(c1, $.a1, $.a2) |
+-------------------------------------+
| NULL                                |
| NULL                                |
+-------------------------------------+
2 rows in set (0.00 sec)

mysql> select json_extract_string(c1, '$.a1') from test_123;
+-------------------------------+
| json_extract_string(c1, $.a1) |
+-------------------------------+
| NULL                          |
| test                          |
+-------------------------------+
2 rows in set (0.00 sec)