doc: deprecate coercion to integer in `process.exitCode`

This warns of invalid uses of process.exitCode (such as mentioned in https://github.com/nodejs/node/pull/43716#issuecomment-1242824575).

In a quick search for process.exitCode below, I can't find invalid uses. However, I open this PR since it may need the doc-only deprecation first instead of throwing a validation error.

• process.exitCode =
• process.exitCode = undefined
  • The doc says that the type of process.exitCode is integer, but it should be integer|undefined. Its default value is undefined.

Refs: https://github.com/nodejs/node/pull/43716 Refs: https://github.com/nodejs/node/pull/43738

Signed-off-by: Daeyeon Jeong daeyeon.dev@gmail.com