Provide better documentation for `cxx_test` using `gtest`

Created by: sdwilsh

This incorporates the changes suggested by @milch into the docs.

Closes https://github.com/facebook/buck/issues/448

Test Plan: Loaded these pages:

• http://localhost:9811/rule/cxx_test.html
• http://localhost:9811/concept/buckconfig.html#cxx.gtest_dep