MINOR: [Format] Fix incorrect bitmask examples and typos in Flight SQL and format

[HyukjinKwon]

Rationale for this change

968e6ea added incorrect bitmask examples for SQL_SUPPORTED_TRANSACTIONS_ISOLATION_LEVELS.

arrow/format/FlightSql.proto

Lines 588 to 590 in 968e6ea

	* - return 2 (\b10) => [SQL_TRANSACTION_READ_UNCOMMITTED];
	* - return 3 (\b11) => [SQL_TRANSACTION_NONE, SQL_TRANSACTION_READ_UNCOMMITTED];
	* - return 4 (\b100) => [SQL_TRANSACTION_REPEATABLE_READ];

The bitmask incorrectly mapped bit 2 (value 4 = \b100) to SQL_TRANSACTION_REPEATABLE_READ instead of SQL_TRANSACTION_READ_COMMITTED, and omitted SQL_TRANSACTION_READ_COMMITTED entirely. According to the enum definition:

arrow/format/FlightSql.proto

Lines 820 to 826 in 968e6ea

	enum SqlTransactionIsolationLevel {
	SQL_TRANSACTION_NONE = 0;
	SQL_TRANSACTION_READ_UNCOMMITTED = 1;
	SQL_TRANSACTION_READ_COMMITTED = 2;
	SQL_TRANSACTION_REPEATABLE_READ = 3;
	SQL_TRANSACTION_SERIALIZABLE = 4;
	}

The correct bit-to-enum mapping should be:

• Bit 0 (value 1) = SQL_TRANSACTION_NONE (enum value 0)
• Bit 1 (value 2) = SQL_TRANSACTION_READ_UNCOMMITTED (enum value 1)
• Bit 2 (value 4) = SQL_TRANSACTION_READ_COMMITTED (enum value 2)
• Bit 3 (value 8) = SQL_TRANSACTION_REPEATABLE_READ (enum value 3)
• Bit 4 (value 16) = SQL_TRANSACTION_SERIALIZABLE (enum value 4)

What changes are included in this PR?

• Added SQL_TRANSACTION_READ_COMMITTED properly in the comments.
• SQL positioned commands (presumably copied and pasted from other places) -> SQL transaction isolation levels for SQL_SUPPORTED_TRANSACTIONS_ISOLATION_LEVELS
• SQL positioned commands (presumably copied and pasted from other places) -> SQL UNIONs for SQL_SUPPORTED_UNIONS
• ... and a lot of other similar typo fixes.

Are these changes tested?

No, I did not test as they are comment-only.

Are there any user-facing changes?

No, dev-only.