updating comments for HTTPRequest - immutability

SS_HTTPRequest can be read like an array, e.g. echo $request['a'], but cannot be written like an array, e.g. $request['a'] = 5; Added comment to caution people.

See line 375:
==================================================================
	/**
	 * @ignore
	 */
	public function offsetSet($offset, $value) {}
	
	/**
	 * @ignore
	 */
	public function offsetUnset($offset) {}
==================================================================

Might be good to write something about how you are supposed to modify a request, or what you are supposed to do instead (a redirect?).
Merge pull request #2923 from IgorNadj/patch-1

updating comments for HTTPRequest - immutability