An identifier must begin with a letter or an underscore. (#888)

Author: leto-b

src/UserGuide/Master/Table/SQL-Manual/Identifier.md

@@ -25,7 +25,7 @@ In IoTDB, identifiers are used to represent the names of databases, tables, colu
 
 ## 1. Naming Rules
 
-1. **Starting Character**: An identifier must begin with a letter.
+1. **Starting Character**: An identifier must begin with a letter or an underscore.
 2. **Subsequent Characters**: Identifiers can include letters, digits, and underscores.
 3. **Special Characters**: If an identifier contains characters other than letters, digits, and underscores, it must be enclosed in double quotes (`"`).
 4. **Escape Characters**: In double-quoted identifiers, a double quote character can be represented using two consecutive double quotes (`""`).

src/UserGuide/latest-Table/SQL-Manual/Identifier.md

@@ -25,7 +25,7 @@ In IoTDB, identifiers are used to represent the names of databases, tables, colu
 
 ## 1. Naming Rules
 
-1. **Starting Character**: An identifier must begin with a letter.
+1. **Starting Character**: An identifier must begin with a letter or an underscore.
 2. **Subsequent Characters**: Identifiers can include letters, digits, and underscores.
 3. **Special Characters**: If an identifier contains characters other than letters, digits, and underscores, it must be enclosed in double quotes (`"`).
 4. **Escape Characters**: In double-quoted identifiers, a double quote character can be represented using two consecutive double quotes (`""`).

src/zh/UserGuide/Master/Table/SQL-Manual/Identifier.md

@@ -25,7 +25,7 @@
 
 ## 1. 命名规则
 
-- __开头字符__：标识符必须以字母开头。
+- __开头字符__：标识符必须以字母或下划线开头。
 - __后续字符__：可以包含字母、数字和下划线。
 - __特殊字符__：如果标识符包含除字母、数字和下划线之外的其他字符，必须用双引号（`"`）括起来。
 - __转义字符__：在双引号定界的标识符中，使用两个连续的双引号（`""`）来表示一个双引号字符。

src/zh/UserGuide/latest-Table/SQL-Manual/Identifier.md

@@ -25,7 +25,7 @@
 
 ## 1. 命名规则
 
-- __开头字符__：标识符必须以字母开头。
+- __开头字符__：标识符必须以字母或下划线开头。
 - __后续字符__：可以包含字母、数字和下划线。
 - __特殊字符__：如果标识符包含除字母、数字和下划线之外的其他字符，必须用双引号（`"`）括起来。
 - __转义字符__：在双引号定界的标识符中，使用两个连续的双引号（`""`）来表示一个双引号字符。