Clarify in advanced-usage that the existing 'jdkfile' distribution can be
used to install Early Access (EA) or other unreleased JDK builds not
provided directly by setup-java, by downloading the archive in a prior
step and pointing jdkFile at it. Adds a concrete EA example.
Addresses #612.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>