[1.6.x] Improved documentation for QueryDict.

Backport of 7f4e2ef1e9 from master

django/http/request.py

@@ -265,12 +265,19 @@ class HttpRequest(object):
 
 class QueryDict(MultiValueDict):
     """
-    A specialized MultiValueDict that takes a query string when initialized.
-    This is immutable unless you create a copy of it.
+    A specialized MultiValueDict which represents a query string.
 
-    Values retrieved from this class are converted from the given encoding
+    A QueryDict can be used to represent GET or POST data. It subclasses
+    MultiValueDict since keys in such data can be repeated, for instance
+    in the data from a form with a <select multiple> field.
+
+    By default QueryDicts are immutable, though the copy() method
+    will always return a mutable copy.
+
+    Both keys and values set on this class are converted from the given encoding
     (DEFAULT_CHARSET by default) to unicode.
     """
+
     # These are both reset in __init__, but is specified here at the class
     # level so that unpickling will have valid values
     _mutable = True