Document --kind-type in ccpp_capgen.py CLI/help, and consider reversing the syntax for external kind modules

Description

The --kind-type argument for capgen accepts kind values that are defined in iso_fortran_env, and it also accepts kind_values that are defined in files provided by the user. The latter syntax, however, isn't documented.

The current syntax is, for example

--kind-type kind_phys=kind_r8:my_host_kinds

and

--kind-type kind_phys=real64

for which the code checks `iso_fortran_env.

I am suggesting to reverse the syntax to

--kind-type NAME=[MODULE:]SPEC

to match my suggestion for external ddts in #751 and because it is more intuitive given the corresponding Fortran code:

use my_host_kinds, only: kind_r8

In addition, there is also this metadata attribute that uses MODULE:SPEC in this order:

[ccpp-table-properties]
  name = temp_set
  type = scheme
  source_path = source_dir2
  kind_spec = temp_kinds:kind_temp=>temp_r8
  dependencies = temp_kinds.F90
  dependencies_path = adjust

Not sure whether anyone is using this syntax, though.

Solution

See above

Alternatives (optional)

Don't change the syntax, but update the CLI/capgen help.

Related to (optional)

#751

Start in ccpp_capgen.py by locating the --kind-type argument and its CLI/help text, then compare the documented examples with the current external kind-module syntax. Review the related #751 discussion and the kind_spec metadata example before deciding whether to document the existing form or reverse it. Done means the chosen syntax is consistently documented and the help text matches the behavior.