Repository: nodejs/node
Pull request: doc: explain child_process code and signal null values everywhere

The `child_process` documentation already explains when `code` and `signal` can be `null` in the parameters of [`spawnSync()`](https://nodejs.org/api/child_process.html#child_processspawnsynccommand-args-options) and the description of [`exit` event](https://nodejs.org/api/child_process.html#event-exit). This change now consistently explains this in the `close` and `exit` event parameters and descriptions.

Refs: https://github.com/nodejs/node/issues/58463#issuecomment-2911460454

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Changed files:
filename: doc/api/child_process.md
patch: @@ -1448,15 +1448,22 @@ instances of `ChildProcess`.
 added: v0.7.7
 -->
 
-* `code` {number} The exit code if the child process exited on its own.
-* `signal` {string} The signal by which the child process was terminated.
+* `code` {number} The exit code if the child process exited on its own, or
+  `null` if the child process terminated due to a signal.
+* `signal` {string} The signal by which the child process was terminated, or
+  `null` if the child process did not terminated due to a signal.
 
 The `'close'` event is emitted after a process has ended _and_ the stdio
 streams of a child process have been closed. This is distinct from the
 [`'exit'`][] event, since multiple processes might share the same stdio
 streams. The `'close'` event will always emit after [`'exit'`][] was
 already emitted, or [`'error'`][] if the child process failed to spawn.
 
+If the process exited, `code` is the final exit code of the process, otherwise
+`null`. If the process terminated due to receipt of a signal, `signal` is the
+string name of the signal, otherwise `null`. One of the two will always be
+non-`null`.
+
 ```cjs
 const { spawn } = require('node:child_process');
 const ls = spawn('ls', ['-lh', '/usr']);
@@ -1526,8 +1533,10 @@ See also [`subprocess.kill()`][] and [`subprocess.send()`][].
 added: v0.1.90
 -->
 
-* `code` {number} The exit code if the child process exited on its own.
-* `signal` {string} The signal by which the child process was terminated.
+* `code` {number} The exit code if the child process exited on its own, or
+  `null` if the child process terminated due to a signal.
+* `signal` {string} The signal by which the child process was terminated, or
+  `null` if the child process did not terminated due to a signal.
 
 The `'exit'` event is emitted after the child process ends. If the process
 exited, `code` is the final exit code of the process, otherwise `null`. If the