docs: missing documentation for XOR and IS_TRUE boolean scalar functions

[sundy-li]

What's Missing

Two boolean scalar functions registered in the Databend function registry have no documentation:

• XOR(a, b) — returns the logical exclusive-or of two boolean values
• IS_TRUE(expr) — returns TRUE if the expression is TRUE (not NULL, not FALSE)

Source File

/workspace/databend/src/query/functions/src/scalars/boolean.rs

Relevant registrations:

registry.register_passthrough_nullable_2_arg::<BooleanType, BooleanType, BooleanType, _, _>(
    "xor",
    ...
);
registry.register_1_arg_core::<BooleanType, BooleanType, _, _>(
    "is_true",
    ...
);
registry.register_1_arg_core::<NullableType<BooleanType>, BooleanType, _, _>(
    "is_true",
    ...
);

What They Do

• XOR(a, b) — logical XOR: returns TRUE when exactly one of the two boolean arguments is TRUE. Both arguments are non-nullable booleans.
• IS_TRUE(expr) — returns TRUE only when the input is a non-NULL TRUE value. Accepts both nullable and non-nullable booleans, always returns a non-nullable boolean. Useful for safely testing nullable boolean columns without NULL propagation.

Suggested Doc Location

/docs/en/sql-reference/20-sql-functions/03-conditional-functions/ (alongside if.md, iff.md, is-null.md, etc.)

Both functions could be documented in a single page xor-is-true.md or as two separate pages xor.md and is-true.md.