doc: update options for exec and execFile

The stdio and customFds options are never used by exec or execFile,
remove them from the documentation for these functions.

doc/api/child_process.markdown

@@ -466,11 +466,6 @@ See also: `child_process.exec()` and `child_process.fork()`
 * `command` {String} The command to run, with space-separated arguments
 * `options` {Object}
   * `cwd` {String} Current working directory of the child process
-  * `stdio` {Array|String} Child's stdio configuration. (See above)
-    Only stdin is configurable, anything else will lead to unpredictable
-    results.
-  * `customFds` {Array} **Deprecated** File descriptors for the child to use
-    for stdio.  (See above)
   * `env` {Object} Environment key-value pairs
   * `encoding` {String} (Default: 'utf8')
   * `timeout` {Number} (Default: 0)
@@ -524,9 +519,6 @@ the child process is killed.
 * `args` {Array} List of string arguments
 * `options` {Object}
   * `cwd` {String} Current working directory of the child process
-  * `stdio` {Array|String} Child's stdio configuration. (See above)
-  * `customFds` {Array} **Deprecated** File descriptors for the child to use
-    for stdio.  (See above)
   * `env` {Object} Environment key-value pairs
   * `encoding` {String} (Default: 'utf8')
   * `timeout` {Number} (Default: 0)