Update docs for language yaml configs

lib/linguist/content_types.yml

@@ -1,3 +1,5 @@
+# MIME types and extensions to override when the raw blob is served
+
 # mime types
 application/rdf+xml: text/plain
 application/sh: text/plain

lib/linguist/languages.yml

@@ -1,6 +1,8 @@
-# This file maps file extensions and exact file names to languages.
+# Defines all Lanaguges known to GitHub
-# If the key begins with a `.`, it's an extension.
+#
-# Anything else will only match exactly, albeit case insensitively.
+# lexer - An explicit lexer String (defaults to name.downcase)
+# ext   - An Array of associated extensions
+
 ASP:
   :lexer: aspx-vb
   :ext: