lib: correct deprecated db open functions' docs

lib: correct deprecated db open functions' docs

Both notmuch_database_open() and notmuch_database_open_verbose()'s
documentation state they call notmuch_database_open_with_config() with
config_path=NULL; however, their implementations pass an empty string.
The empty string is the correct value to maintain their original
behavior of not loading the user's configuration so their documentation
is incorrect.