doc: add more details to process.env

process.env has a few quirks that deserve documenting.

The commit documents:

- How assigning to process.env will implicitly call `toString()`
- How to remove an environment variable from process.env

PR-URL: https://github.com/nodejs/node/pull/4924
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/process.markdown

@@ -389,6 +389,31 @@ But this will:
     process.env.foo = 'bar';
     console.log(process.env.foo);
 
+Assigning a property on `process.env` will implicitly convert the value
+to a string.
+
+Example:
+
+```js
+process.env.test = null;
+console.log(process.env.test);
+// => 'null'
+process.env.test = undefined;
+console.log(process.env.test);
+// => 'undefined'
+```
+
+Use `delete` to delete a property from `process.env`.
+
+Example:
+
+```js
+process.env.TEST = 1;
+delete process.env.TEST;
+console.log(process.env.TEST);
+// => undefined
+```
+
 ## process.execArgv
 
 This is the set of Node.js-specific command line options from the