Add mapping of untyped nil args to BindNull to exec docs

[davidjb]

In my app, values passed in from JSON (and elsewhere) where they could be a value or NULL in SQL are stored as typed pointers, such as *string. In order to ensure these are handled by sqlitex and bound as BindNull, the nil value needs to be passed as an untyped nil, rather than a *string

This needs to be handled on my side so this PR documents the handling of untyped nil to BindNull in the ExecOptions documentation, alongside how other types are handled.

This also slightly adjusts the default handling documentation, so each section mentions fmt.Sprint(v) and aligns with the implementation (https://github.com/zombiezen/go-sqlite/blob/main/sqlitex/exec.go#L327).

[zombiezen]

Sorry to leave you hanging for so long. I'm not sure how this slipped through the cracks on my end. A few minor tweaks, please:

[davidjb]

Thanks for the review @zombiezen. I've updated the PR and fixed the inconsistent spacing (tabs/spaces). This is what it looks like now when rendered:

I updated the fmt.Sprintf references in the type struct comments to match the Exec docs; the latter renders as links but the former doesn't so let me know your thoughts.

Let me know if any thing else needs adjusting.

[zombiezen]

LGTM! Thanks.