Rewrite parts of the documentation.

Describe every expect method with a single test.
Revert changes of 128553f. Fixes #687.

Still fixes #121, but the original fix was superfluous. Added documentation
about handling assumes and asserts in conjunction with the
ExpectedException rule.
Extract isAnyExceptionExpected().

Improve readability and avoid duplicate code.
Merge pull request #720 from stefanbirkner/expected-exception

Extend ExpectedException's documentation and fix #687.
Merge branch 'master' of https://github.com/junit-team/junit