doc: Fix XML_CATALOG_FILES env var for Apple Silicon machines

 doc: Fix XML_CATALOG_FILES env var for Apple Silicon machines
 
 Homebrew changed the prefix for Apple Silicon based machines, so
 our advice for XML_CATALOG_FILES needs to mention both.  More info
 on the Homebrew change can be found at:
 
 https://github.com/Homebrew/brew/issues/9177
 
 This is backpatch of commits 4c8d65408 and 5a91c7975, the latter
 which contained a small fix based on a report from Dagfinn Ilmari
 Mannsåker.
 
 Author: Julien Rouhaud <julien.rouhaud@free.fr>
 Discussion: https://postgr.es/m/20230327082441.h7pa2vqiobbyo7rd@jrouhaud