Fix remapped diagnostic documentation

[angus-g]

Hopefully I'm not misremembering/misunderstanding the diag_table format, but the first column should reflect the module names for the diagnostic coordinate systems?

[marshallward]

Looks like you are correct. All the diag_table that I can see have _z for the z-interpolated diagnostics, and this is the line where the module name is assigned:

    new_module_name = trim(module_name)//'_'//trim(diag_cs%diag_remap_cs(i)%diag_module_suffix)

I'll merge this if there's no objections.

[ashao]

I agree with @angus-g's correction to the documentation

[marshallward]

Thanks @ashao

(NOTE: Skipping regression since does not touch any source code)