doc: fixes default shell in child_process.md

Clarifies the default shell in Windows is process.env.ComSpec
and that cmd.exe is only used as a fallback. Functions whose
descriptions are affected include:
child_process.spawn, child_process.exec,
child_process.spawnsSync, and child_process.execSync.

Fixes: nodejs#14156

doc/api/child_process.md

@@ -1,4 +1,4 @@
-# Child Process
+# Child Process
 
 > Stability: 2 - Stable
 
@@ -133,9 +133,10 @@ added: v0.1.90
   * `env` {Object} Environment key-value pairs
   * `encoding` {string} (Default: `'utf8'`)
   * `shell` {string} Shell to execute the command with
-    (Default: `'/bin/sh'` on UNIX, `'cmd.exe'` on Windows, The shell should
-     understand the `-c` switch on UNIX or `/d /s /c` on Windows. On Windows,
-     command line parsing should be compatible with `cmd.exe`.)
+    (Default: `'/bin/sh'` on UNIX, `'process.env.ComSpec'` on Windows. If
+    `'process.env.ComSpec'` is unavailable, uses `'cmd.exe'` on Windows instead.
+    The shell should understand the `-c` switch on UNIX or `/d /s /c` on Windows.
+    On Windows, command line parsing should be compatible with `cmd.exe`.)
   * `timeout` {number} (Default: `0`)
   * `maxBuffer` {number} Largest amount of data in bytes allowed on stdout or
     stderr. (Default: `200*1024`) If exceeded, the child process is terminated.
@@ -382,9 +383,10 @@ changes:
   * `uid` {number} Sets the user identity of the process. (See setuid(2).)
   * `gid` {number} Sets the group identity of the process. (See setgid(2).)
   * `shell` {boolean|string} If `true`, runs `command` inside of a shell. Uses
-    `'/bin/sh'` on UNIX, and `'cmd.exe'` on Windows. A different shell can be
-    specified as a string. The shell should understand the `-c` switch on UNIX,
-    or `/d /s /c` on Windows. Defaults to `false` (no shell).
+    `'/bin/sh'` on UNIX, and `'process.env.ComSpec'` on Windows. If
+    `'process.env.ComSpec'` is unavailable, uses `'cmd.exe'` on Windows instead.
+    A different shell can be specified as a string. The shell should understand
+    the `-c` switch on UNIX, or `/d /s /c` on Windows. Defaults to `false` (no shell).
 * Returns: {ChildProcess}
 
 The `child_process.spawn()` method spawns a new process using the given
@@ -707,9 +709,10 @@ changes:
       `stdio` is specified
   * `env` {Object} Environment key-value pairs
   * `shell` {string} Shell to execute the command with
-    (Default: `'/bin/sh'` on UNIX, `'cmd.exe'` on Windows, The shell should
-     understand the `-c` switch on UNIX or `/d /s /c` on Windows. On Windows,
-     command line parsing should be compatible with `cmd.exe`.)
+    (Default: `'/bin/sh'` on UNIX, `'process.env.ComSpec'` on Windows.
+    If `'process.env.ComSpec'` is unavailable, uses `'cmd.exe'` on Windows instead.
+    The shell should understand the `-c` switch on UNIX or `/d /s /c` on Windows.
+    On Windows, command line parsing should be compatible with `cmd.exe`.)
   * `uid` {number} Sets the user identity of the process. (See setuid(2).)
   * `gid` {number} Sets the group identity of the process. (See setgid(2).)
   * `timeout` {number} In milliseconds the maximum amount of time the process
@@ -775,9 +778,10 @@ changes:
   * `encoding` {string} The encoding used for all stdio inputs and outputs.
     (Default: `'buffer'`)
   * `shell` {boolean|string} If `true`, runs `command` inside of a shell. Uses
-    `'/bin/sh'` on UNIX, and `'cmd.exe'` on Windows. A different shell can be
-    specified as a string. The shell should understand the `-c` switch on UNIX,
-    or `/d /s /c` on Windows. Defaults to `false` (no shell).
+    `'/bin/sh'` on UNIX, and `'process.env.ComSpec'` on Windows. If
+    `'process.env.ComSpec'` is unavailable, uses `'cmd.exe'` on Windows instead.
+    A different shell can be specified as a string. The shell should understand
+    the `-c` switch on UNIX, or `/d /s /c` on Windows. Defaults to `false` (no shell).
 * Returns: {Object}
   * `pid` {number} Pid of the child process
   * `output` {Array} Array of results from stdio output