docs: document virtio-blk discard support

Document the supported discard configurations, advertised alignment
rules, and user responsibility for compatible block device updates. Add
the feature to the changelog.

[jackabt@amazon.com: rebased onto main and moved the changelog entry
to the unreleased section]

Signed-off-by: Jonas Savulionis <jonas@esnet.lt>
Signed-off-by: Rekas <aure369@gmail.com>
Signed-off-by: Jack Thomson <jackabt@amazon.com>
Signed-off-by: Pierre Bertholom <pbertho@amazon.com>