Added support for stream-like downloads of existing files to force_download()

Based on code/ideas from PR #365, #1254
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 6047f1c..e523c4c 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -77,8 +77,10 @@
    -  :doc:`Inflector Helper <helpers/inflector_helper>` changes include:
 	 - Changed :php:func:`humanize()` to allow passing an input separator as its second parameter.
 	 - Refactored :php:func:`plural()` and :php:func:`singular()` to avoid double pluralization and support more words.
-   -  Added an optional third parameter to ``force_download()`` that enables/disables sending the actual file MIME type in the Content-Type header (disabled by default).
-   -  Added a work-around in ``force_download()`` for a bug Android <= 2.1, where the filename extension needs to be in uppercase.
+   -  :doc:`Download Helper <helpers/download_helper>` changes include:
+	 - Added an optional third parameter to :php:func:`force_download()` that enables/disables sending the actual file MIME type in the Content-Type header (disabled by default).
+	 - Added a work-around in :php:func:`force_download()` for a bug Android <= 2.1, where the filename extension needs to be in uppercase.
+	 - Added support for reading from an existing file path by passing NULL as the second parameter to :php:func:`force_download()` (useful for large files and/or safely transmitting binary data).
    -  :doc:`Form Helper <helpers/form_helper>` changes include:
 	 - :php:func:`form_dropdown()` will now also take an array for unity with other form helpers.
 	 - :php:func:`form_prep()`'s second argument now only accepts a boolean value, which determines whether the value is escaped for a <textarea> or a regular <input> element.