Document limitation of normal map-style constructor use when using the Immutable annotation.

See http://groovy.329449.n5.nabble.com/Unexpected-behaviour-when-using-Immutable-on-a-class-with-java-util-Map-field-tp5716170.html
Merge pull request #219 from jwagenleitner/immutable-javadoc

Document limitation of normal map-style constructor use when using the Immutable annotation