Bug 6152: Document DOM authority index config

Bug 6152: Document DOM authority index config

Add a note so developers know not to edit authority-zebra-indexdefs.xsl directly
but to instead edit authority-koha-indexdefs.xml and use the
koha-indexdefs-to-zebra.xsl stylesheet to generate the former from the latter.

Signed-off-by: Frederic Demians <frederic@tamil.fr>
Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
(cherry picked from commit 190a4234f7e4a869498c6cc67453bbe8a256467c)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>