This commit fixes incorrect references to `AggregateItemReader#__$$BEGIN_RECORD$$__`
and `AggregateItemReader#__$$END_RECORD$$__` by updating the description of the
reader with the one in its Javadoc.
It also adds a note that the AggregateItemReader is not part of the standard library
of readers provided by Spring Batch but only given as a sample.
Issue #1793
This commit migrates the old Spring Batch documentation to a new
asciidoc toolchain. It will be the first piece in modernizing the
existing Spring Batch documentation. Future steps will include making
java based configuraion more prominant in the reference documentation.
Resolves BATCH-2620