doc: update the deprecation for exit code to clarify its scope

[daeyeon]

This updates the deprecation, DEP0164, to clarify its scope.

Previously, process.exitCode wasn't mentioned but it needs to be applied with the same deprecation because its meaning is the same as the code parameter and it's overridden with the code value in process.exit().

Refs: https://github.com/nodejs/node/pull/44712 Refs: https://github.com/nodejs/node/pull/43738

Signed-off-by: Daeyeon Jeong [email protected]