update ABNF form im MKtl_description_files

[adcxyz]

MKtl_description_files has a definition of the description file format in ABNF form.
This should be updated to the current desc file format.

https://github.com/ModalityTeam/Modality-toolkit/blob/master/Modality/HelpSource/Reference/MKtl_description_files.schelp

[LFSaw]

what is ABNF (I know…)? Why do we need it here? I'd vote for removing it. I never looked at it, did anyone?

[LFSaw]

I remeber @miguel-negrao working that one out... maybe he can comment?

[miguel-negrao]

ABNF is a standard formal definition of a language. It allows to specify with precision text which is considered correct according to a certain language. In our case, it allows to specify what is a a correct definition file.

[miguel-negrao]

It might be too much trouble to keep it up to date. Perhaps better to delete it and just use a more ad-hoc method of specifying the syntax.