Update assets module with template docstring.

Mod:
    bitshift/assets.py
        -convert existing docstrings to the Sphinx auto-doc format.

bitshift/assets.py

@@ -1,6 +1,6 @@
 """
-Module contains helper functions to be used inside the project's Jinja
-templates.
+.. module:: assets
+   :synopsis: Helper functions for use inside the project's Jinja templates.
 """
 
 from flask import Markup
@@ -12,10 +12,12 @@ ASSET_HTML_TEMPLATES = {
 
 def tag(filename):
     """
-    Return HTML tag for asset named filename.
+    Generate an HTML tag for a CSS/JS asset, based on its file extension.
 
-    Return either a <script> or <link> tag to the file named filename,
-    based on its extension.
+    :param filename: The filename of the asset to create a tag for.
+
+    :return: A string containing a `<source>` tag for JS files, and a `<link>`
+        for CSS files.
     """
 
     file_ext = filename.split(".")[-1]