ISA: Signed MOD clarifications

[dthaler]

References:

• https://lore.kernel.org/bpf/DM4PR21MB3440CDB9D8E325CBEA20FFA7A3569@DM4PR21MB3440.namprd21.prod.outlook.com/
• https://lore.kernel.org/bpf/20230929210317.g10pla8fTxtwoMvzJDfdt-7QZW4Ni630SWhivXhbnDw@z/

[dthaler]

Original proposed text and rationale:

https://lore.kernel.org/bpf/DM4PR21MB34402522B614257706D2F785A3569@DM4PR21MB3440.namprd21.prod.outlook.com/

[dthaler]

Fixed in PR #58