[UIMA-3854] Update migration section in documentation
- Reorganized migration section
- Fixed some typos
- Added section for 2.0.0 -> 2.1.0


git-svn-id: https://svn.apache.org/repos/asf/uima/uimafit/branches/2.1.x@1600362 13f79535-47bb-0310-9956-ffa450edef68
[UIMA-3559] JDK 8 complains about bad JavaDoc
- Override maven-plugin-plugin version to 3.3 which has MPLUGIN-244 fixed

git-svn-id: https://svn.apache.org/repos/asf/uima/uimafit/branches/2.1.x@1600369 13f79535-47bb-0310-9956-ffa450edef68