doc: remove all references to setsid

Fixes #2299.

doc/api/child_process.markdown

@@ -119,7 +119,6 @@ See `child_process.fork()` for details.
   * `customFds` {Array} **Deprecated** File descriptors for the child to use
     for stdio.  (See below)
   * `env` {Object} Environment key-value pairs
-  * `setsid` {Boolean}
 * return: {ChildProcess object}
 
 Launches a new process with the given `command`, with  command line arguments in `args`.
@@ -228,7 +227,6 @@ See also: `child_process.exec()` and `child_process.fork()`
   * `customFds` {Array} **Deprecated** File descriptors for the child to use
     for stdio.  (See below)
   * `env` {Object} Environment key-value pairs
-  * `setsid` {Boolean}
   * `encoding` {String} (Default: 'utf8')
   * `timeout` {Number} (Default: 0)
   * `maxBuffer` {Number} (Default: 200*1024)
@@ -285,7 +283,6 @@ the child process is killed.
   * `customFds` {Array} **Deprecated** File descriptors for the child to use
     for stdio.  (See below)
   * `env` {Object} Environment key-value pairs
-  * `setsid` {Boolean}
   * `encoding` {String} (Default: 'utf8')
   * `timeout` {Number} (Default: 0)
   * `maxBuffer` {Number} (Default: 200*1024)
@@ -310,7 +307,6 @@ leaner than `child_process.exec`. It has the same options.
   * `customFds` {Array} **Deprecated** File descriptors for the child to use
     for stdio.  (See below)
   * `env` {Object} Environment key-value pairs
-  * `setsid` {Boolean}
   * `encoding` {String} (Default: 'utf8')
   * `timeout` {Number} (Default: 0)
 * Return: ChildProcess object