add comments

git-svn-id: http://h2database.googlecode.com/svn/trunk@5136 e6896862-9d19-0410-bfb3-e9fa5d50e656
avoid copying data from ByteBuffer to another ByteBuffer if possible,  specially for the OffHeapStore

git-svn-id: http://h2database.googlecode.com/svn/trunk@5137 e6896862-9d19-0410-bfb3-e9fa5d50e656