inconsistent multithread (`-x` option) behavior about gmt

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[joa-quim]

OK, I've worked a bit on this and ended up with #8523 One of the reasons for the inconsistency is that while most parallelized codes use OpenMP, grdfilter uses gthreads and followed a different development path. I now make all cores by default in #8523. This still leaves out the gmt & grdgravmag3d modules in the potential supplement because the tests were showing conflicting results between the runs in my machine and those of the CI tests.

The issue with surface showing the -x option on the online help was an error. There was an attempt to OpenMP 'ize surface but, if I still remember, there were some small differences that were never resolved and that code still lives in the repository but is not in use. Maybe it was from there that the -x option was advertised but it was never really implemented. I removed it #8523.

Also clarified the -x in explain_core.rst.

Let's hope that the PR is good. For the moment Mac seems to be still resisting.

[joa-quim]

Addressed bu #8523