Explain longoptions in the documentation

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.

Start by reviewing the _3.General Features_ documentation and the existing material comparing classic and modern syntax. Compare how long options should be represented there and on each module’s page, then agree on the documentation structure before editing. Done means the agreed approach is applied consistently before the GMT6.6 release.

Written by the indexing model from the issue text.