* lib/minitest/mock.rb: nodoc top-level module

Merging r39020 from ruby-trunk

There is some wonkyness going on with the MiniTest docs, see ruby-doc.org:

* [benchmark](http://ruby-doc.org/stdlib-2.0/libdoc/minitest/benchmark/rdoc/index.html)
* [mock](http://ruby-doc.org/stdlib-2.0/libdoc/minitest/mock/rdoc/index.html)
* [spec](http://ruby-doc.org/stdlib-2.0/libdoc/minitest/spec/rdoc/index.html)
* [unit](http://ruby-doc.org/stdlib-2.0/libdoc/minitest/unit/rdoc/index.html)

MiniTest::Unit is the only one that looks decent because it includes README.txt

I realize you have your own docs generated on seattlerb.org, but a lot of people use ruby-doc.org, it's the third result for ["minitest documentation"](https://www.google.com/search?q=minitest+documentation).