Clarify changelog entry.

`expect_with :stdlib` is not supported in RSpec 3;
it's `expect_with :minutest` or `expect_with :test_unit` now.

[ci skip]
Reword sentence for greater clarity.

I originally took "it will fail to communicate" to mean
that it won't communicate rather than meaning the example
will be marked as failed in order to communicate.

[ci skip]
Use `RSpec.describe`.

This is the "primary" entry point in rspec 3 rather
than the monkey-patched `main.describe`

[ci skip]
Merge pull request #1518 from rspec/doc-improvements-2014-05-06

Doc improvements