Document that -J is not required anymore for modern mode?

[Esteban82]

As I understand it, -J is an optional argument for all GMT modules. And from what I see in the following code, the default values are defined by these lines in src/gmt_inti.c:

https://github.com/GenericMappingTools/gmt/blob/82a5564e94777b4e79219801a443a995d015c389/src/gmt_init.c#L16127-L16148

However, I cannot find this mentioned in the documentation anywhere. Is this the case? Or is there any good reason not to do it? I offer to document it if needed.

This is related #8694.