Add some function doc (#1377)

docs/documentation/cn/sql-reference/sql-functions/string-functions/left.md

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+# left
+
+## Syntax
+
+`VARCHAR left(VARCHAR str)`
+
+## Description
+
+它返回具有指定长度的字符串的左边部分
+
+## Examples
+
+```
+mysql> select left("Hello doris",5);
++------------------------+
+| left('Hello doris', 5) |
++------------------------+
+| Hello                  |
++------------------------+
+```

docs/documentation/cn/sql-reference/sql-functions/string-functions/right.md

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+# right
+
+## Syntax
+
+`VARCHAR right(VARCHAR str)`
+
+## Description
+
+它返回具有指定长度的字符串的右边部分
+
+## Examples
+
+```
+mysql> select right("Hello doris",5);
++-------------------------+
+| right('Hello doris', 5) |
++-------------------------+
+| doris                   |
++-------------------------+
+```

docs/documentation/cn/sql-reference/sql-functions/string-functions/strleft.md

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+# strleft
+
+## Syntax
+
+`VARCHAR strleft(VARCHAR str)`
+
+## Description
+
+它返回具有指定长度的字符串的左边部分
+
+## Examples
+
+```
+mysql> select strleft("Hello doris",5);
++------------------------+
+| strleft('Hello doris', 5) |
++------------------------+
+| Hello                  |
++------------------------+
+```

docs/documentation/cn/sql-reference/sql-functions/string-functions/strright.md

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+# strright
+
+## Syntax
+
+`VARCHAR strright(VARCHAR str)`
+
+## Description
+
+它返回具有指定长度的字符串的右边部分
+
+## Examples
+
+```
+mysql> select strright("Hello doris",5);
++-------------------------+
+| strright('Hello doris', 5) |
++-------------------------+
+| doris                   |
++-------------------------+
+```