Explain longoptions in the documentation

[Esteban82]

We have to update the documentation to include the long options (which are already available in the code).

I think we should do it before the release of GMT6.6 (#8653).

I am not sure how we should include them in the documentation. No doubt we should add a section in 3.General Features.

But for example, I don't know what should be done for the pages of each module. Whether to add a table with the conversion. Or write another page using only the long options (similar to what was done with the classic vs. modern syntax). What do you think?

I think we should discuss and agree on how we should update the documentation.

[seisman]

I think few people will use long options if we can't provide a way for autocompletion.

[joa-quim]

I'm afraid I'll have to agree. But how could we do that? Have no idea on how to do it in Windows Cmd. Only thing that comes to mind is some kind of VSC plugin, but requiring users to use VSC to write one-liners is a no-no.