Make the example functions self-documenting in duckdb_functions()

[rustyconover]

Why

I loaded all 346 extensions in the community-extensions catalog and diffed duckdb_functions() before and after LOAD:

Distinct function names registered	5,483
...carrying a description	29.6%
Function-registering extensions exposing zero descriptions	80.7% (201 of 249)
Functions whose parameters are col0, col1 rather than real names	51%
Settings registered by those same extensions	496 — 100% documented

That last row is the interesting one. Settings hit 100% not because their authors are more diligent, but because DBConfig::AddExtensionOption(name, description, ...) takes the description as a required positional argument — you cannot register an undocumented setting. Meanwhile loader.RegisterFunction(ScalarFunction), the overload this template uses and therefore the one nearly every extension uses, has nowhere to put a description at all.

The template is the highest-leverage place to fix this: it's what almost every extension author starts from.

What changed

RegisterFunction(ScalarFunction) becomes RegisterFunction(CreateScalarFunctionInfo) with a FunctionDescription attached.

// before
auto waddle_scalar_function =
    ScalarFunction("waddle", {LogicalType::VARCHAR}, LogicalType::VARCHAR, WaddleScalarFun);
loader.RegisterFunction(waddle_scalar_function);

// after
CreateScalarFunctionInfo info(
    ScalarFunction("waddle", {LogicalType::VARCHAR}, LogicalType::VARCHAR, WaddleScalarFun));

FunctionDescription desc;
desc.parameter_names = {"name"};
desc.description = "Returns `name` prefixed with a waddling duck.";
desc.examples = {"waddle('Jane')"};
desc.categories = {"string"};
info.descriptions.push_back(std::move(desc));

// ALTER_ON_CONFLICT adds an overload if a function with this name already exists
info.on_conflict = OnCreateConflict::ALTER_ON_CONFLICT;
loader.RegisterFunction(std::move(info));

on_conflict is set explicitly because that's what RegisterFunction(ScalarFunction) does internally; without it the registration would default to ERROR_ON_CONFLICT and the template's behaviour would quietly change.

Three files, no submodule pin changes:

• src/waddle_extension.cpp — both example functions now register with a description, parameter names, an example and a category.
• test/sql/waddle.test — asserts the metadata reaches the catalog, including that parameters[1] is name and not col0.
• docs/README.md — a short ### Documenting your functions subsection.

CreateAggregateFunctionInfo and CreateTableFunctionInfo carry the same descriptions field; the README says so, though the template has no aggregate or table function example to convert.

Payoff

duckdb_functions() stops being a list of names and becomes a usable interface. Anything introspecting a DuckDB instance to discover what it can call — a user who just ran INSTALL foo FROM community, tooling, or an agent — gets parameter names, a one-line summary and a runnable example instead of col0 and NULL.