doc: updated default env on child_process funcs

doc/api/child_process.md

@@ -142,7 +142,7 @@ changes:
 * `options` {Object}
   * `cwd` {string} Current working directory of the child process.
     **Default:** `null`.
-  * `env` {Object} Environment key-value pairs. **Default:** `null`.
+  * `env` {Object} Environment key-value pairs. **Default:** `process.env`.
   * `encoding` {string} **Default:** `'utf8'`
   * `shell` {string} Shell to execute the command with. See
     [Shell Requirements][] and [Default Windows Shell][]. **Default:**
@@ -245,7 +245,7 @@ changes:
 * `args` {string[]} List of string arguments.
 * `options` {Object}
   * `cwd` {string} Current working directory of the child process.
-  * `env` {Object} Environment key-value pairs.
+  * `env` {Object} Environment key-value pairs. **Default:** `process.env`.
   * `encoding` {string} **Default:** `'utf8'`
   * `timeout` {number} **Default:** `0`
   * `maxBuffer` {number} Largest amount of data in bytes allowed on stdout or
@@ -335,7 +335,7 @@ changes:
   * `detached` {boolean} Prepare child to run independently of its parent
     process. Specific behavior depends on the platform, see
     [`options.detached`][]).
-  * `env` {Object} Environment key-value pairs.
+  * `env` {Object} Environment key-value pairs.  **Default:** `process.env`.
   * `execPath` {string} Executable used to create the child process.
   * `execArgv` {string[]} List of string arguments passed to the executable.
     **Default:** `process.execArgv`.
@@ -400,7 +400,7 @@ changes:
 * `args` {string[]} List of string arguments.
 * `options` {Object}
   * `cwd` {string} Current working directory of the child process.
-  * `env` {Object} Environment key-value pairs.
+  * `env` {Object} Environment key-value pairs. **Default:** `process.env`.
   * `argv0` {string} Explicitly set the value of `argv[0]` sent to the child
     process. This will be set to `command` if not specified.
   * `stdio` {Array|string} Child's stdio configuration (see
@@ -714,7 +714,7 @@ changes:
   * `stdio` {string|Array} Child's stdio configuration. `stderr` by default will
     be output to the parent process' stderr unless `stdio` is specified.
     **Default:** `'pipe'`.
-  * `env` {Object} Environment key-value pairs.
+  * `env` {Object} Environment key-value pairs.  **Default:** `process.env`.
   * `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