How to specify external derived types

For the [jonquil](https://github.com/toml-f/jonquil) docs I want to link to two types from the TOML Fortran documentation (https://toml-f.github.io/toml-f/type/toml_visitor.html and https://toml-f.github.io/toml-f/type/abstract_lexer.html). How can I instruct FORD to find those?

The extra modules I already specified are:

````toml
[extra.ford.extra_mods]
iso_fortran_env = "https://gcc.gnu.org/onlinedocs/gfortran/ISO_005fFORTRAN_005fENV.html"
tomlf = "https://toml-f.github.io/toml-f/module/tomlf.html"
tomlf_build = "https://toml-f.github.io/toml-f/module/tomlf_build.html"
tomlf_constants = "https://toml-f.github.io/toml-f/module/tomlf_constants.html"
tomlf_datetime = "https://toml-f.github.io/toml-f/module/tomlf_datetime.html"
tomlf_de_abc = "https://toml-f.github.io/toml-f/module/tomlf_de_abc.html"
tomlf_de_context = "https://toml-f.github.io/toml-f/module/tomlf_de_context.html"
tomlf_de_parser = "https://toml-f.github.io/toml-f/module/tomlf_de_parser.html"
tomlf_de_token = "https://toml-f.github.io/toml-f/module/tomlf_de_token.html"
tomlf_error = "https://toml-f.github.io/toml-f/module/tomlf_error.html"
tomlf_diagnostic = "https://toml-f.github.io/toml-f/module/tomlf_diagnostic.html"
tomlf_type = "https://toml-f.github.io/toml-f/module/tomlf_type.html"
tomlf_utils = "https://toml-f.github.io/toml-f/module/tomlf_utils.html"
tomlf_version = "https://toml-f.github.io/toml-f/module/tomlf_version.html"
````

Currently I get the following warning when running ford:

````
Warning: Could not find base type ('abstract_lexer') of derived type
'json_lexer' (in 'lexer.f90').
If 'abstract_lexer' is defined in an external module, you may be able
to tell Ford about its documentation
with the `extra_mods` setting
Warning: Could not find base type ('toml_visitor') of derived type
'json_serializer' (in 'ser.f90').
If 'toml_visitor' is defined in an external module, you may be able to
tell Ford about its documentation
with the `extra_mods` setting
Warning: Could not find base type ('toml_visitor') of derived type 'json_prune'
(in 'parser.f90').
If 'toml_visitor' is defined in an external module, you may be able to
tell Ford about its documentation
with the `extra_mods` setting
````