Added panic message to docs.

[SeeRightThroughMe]

Which issue does this PR close?

N/A

Rationale for this change

Schema::field does not indicate that it panics in the current docs. It does panic though (not that this is particularly surprising!).

Here's an example that will panic:

use arrow::datatypes::SchemaBuilder;
fn main() {
    let mut builder = SchemaBuilder::new();
    let schema = builder.finish();
    let my_field = schema.field(2); // <-- Panics here because index OOB. 
}

What changes are included in this PR?

Just a docstring.

Are there any user-facing changes?

Nope, but perhaps in future, field should either return a result, or we add a try_field function that returns a result (to prevent people needing to change code relying on Schema::field already)

[SeeRightThroughMe]

(also, I know it's a tiny change but this is my first contribution on any open-source project ever! 😄 )

[paddyhoran]

Thank you @SeeRightThroughMe. Congrats on your first contribution 😄

[alamb]

Thanks @paddyhoran and @SeeRightThroughMe