gradle: add comments to the settings for all projects

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: adjust default javadoc member level to PUBLIC

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
gradle: rename index properties to better reflect OBR and R5 settings

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
dist: gradle: adjust to renamed index properties

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>