Improve Javadoc of Memoizer.java

Proper peveloper documentation on the usage of the `Memoizer` may come as we approach 5.1.0.In the meantime, this PR expands the Javadoc for some methods including the new in-place constructor.

To review this PR, check the content of http://ci.openmicroscopy.org/job/BIOFORMATS-5.1-merge-build/javadoc/loci/formats/Memoizer.html

--no-rebase