common : Update the docs on -t --threads (#16236)
* Update the docs on -t --threads
* Revert "Update the docs on -t --threads"
This reverts commit eba97345e2c88d8ca510abec87d00bf6b9b0e0c2.
* docs: clarify -t/--threads parameter uses CPU threads and defaults to all available cores
* Update arg.cpp