Chore: Clarify where table functions run

[scsmithr]

Description

I'm currently running through adding some table funcs, and I'm unsure what gets run where.

The full interface:

#[async_trait]
pub trait TableFunc: Sync + Send {
    /// The name for this table function. This name will be used when looking up
    /// function implementations.
    fn name(&self) -> &str;
    fn runtime_preference(&self) -> RuntimePreference;
    fn detect_runtime(
        &self,
        _args: &[FuncParamValue],
        _parent: RuntimePreference,
    ) -> Result<RuntimePreference> {
        Ok(self.runtime_preference())
    }

    /// Return a table provider using the provided args.
    async fn create_provider(
        &self,
        ctx: &dyn TableFuncContextProvider,
        args: Vec<FuncParamValue>,
        opts: HashMap<String, FuncParamValue>,
    ) -> Result<Arc<dyn TableProvider>>;
    /// Return the signature for this function.
    /// Defaults to None.
    // TODO: Remove the default impl once we have `signature` implemented for all functions
    fn signature(&self) -> Option<Signature> {
        None
    }
}

From what I'm reading, create_provider always gets run remotely if runtime preference is remote. If this is the case, then that should be documented.

[scsmithr]

Also not sure what detect_runtime is for.

[vrongmeal]

Taking a look at this. Will document/refactor as required.