[docs] Improve the doc and indexing of adict.__missing__. (issue 23006)

[JohnJPosner at verizon.net]

The term "subdict" at line 1910 of datamodel.rst seems to be a shorthand
way of saying "instance of a dict subclass", but that's kind of obscure.
The term "subdict" does not appear anywhere else in this document.

I suggest changing "subdict" to "dictionary".

http://bugs.python.org/review/23006/