Add supplementary Javadoc

Includes
- AssertionFailedError
- TestFailure
Merge pull request #914 from mananshah99/master

Add supplementary Javadoc
Description.getMethodName() should not return null. Fixes #892
Merge pull request #903 from baev/master

Description.getMethodName() should not return null. Fixes #892
Improve readability of ComparisonFailure
Mention about using assumptions in Theories
Merge pull request #920 from AlexYursha/improve-assumptions-docs

Improve documentation on using assumptions.
Merge pull request #919 from AlexYursha/comparison-failure

Improve readability of ComparisonFailure