[PATCH] lib: update documentation for notmuch_database_get_directory

[David Bremner]

David Bremner <david at tethera.net> writes:

> Clarify that using the directory after destroying the corresponding
> database is not permitted.
>
> This is implicit in the description of notmuch_database_destroy, but
> it doesn't hurt to be explicit, and we do express similar "ownership"
> relationships at other places in the docs.

pushed,

d