Document `table(scheduled(at = col_name))` syntax

From our macro code:

let scheduled_at_column = scheduled_at_column.ok_or_else(|| {
    syn::Error::new(
        sched.span,
        "scheduled tables must have a `scheduled_at: spacetimedb::ScheduleAt` column. \
                 if the column has a name besides `scheduled_at`, you can specify it with \
                 `scheduled(my_reducer, at = custom_scheduled_at)`",
    )
})?;

This syntax, with the at = col_name parameter supplied, does not appear in our docs, and is not regression-tested. Expand the docs of scheduled reducers, which are attached to the spacetimedb_bindings_table re-export in the bindings crate's lib.rs.

Start in the bindings crate's lib.rs, where the scheduled reducer documentation is attached to the spacetimedb_bindings_table re-export. Document the table(scheduled(at = col_name)) form and review the existing scheduled reducer examples. Add regression coverage for the custom scheduled_at column syntax, and consider the work done when both the docs and behavior are covered.