doc: document process.env better

Fixes nodejs#6424.

Reviewed-By: Fedor Indutny <[email protected]>

doc/api/process.markdown

@@ -276,6 +276,29 @@ Returns the current working directory of the process.
 
 An object containing the user environment. See environ(7).
 
+An example of this object looks like:
+
+    { TERM: 'xterm-256color',
+      SHELL: '/usr/local/bin/bash',
+      USER: 'maciej',
+      PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
+      PWD: '/Users/maciej',
+      EDITOR: 'vim',
+      SHLVL: '1',
+      HOME: '/Users/maciej',
+      LOGNAME: 'maciej',
+      _: '/usr/local/bin/node' }
+
+You can write to this object, but changes won't be reflected outside of your
+process. That means that the following won't work:
+
+    node -e 'process.env.foo = "bar"' && echo $foo
+
+But this will:
+
+    process.env.foo = 'bar';
+    console.log(process.env.foo);
+
 
 ## process.exit([code])