default.conf example for COPY_AS_IS_EXCLUDE_TSM (#3212)

In default.conf explain how one could reduce
the size of the ReaR recovery system initrd
in particular on POWER architecture
via COPY_AS_IS_TSM and alternatively
via COPY_AS_IS_EXCLUDE_TSM, see
https://github.com/rear/rear/issues/3189
and describe the about 123 MiB initrd size limit on POWER
(128 MiB minus about 4 MiB for additional prep boot data), see
https://github.com/rear/rear/pull/3212#issuecomment-2092481874
Also tell about generic methods to reduce the size of the initrd
like MODULES=( 'loaded_modules' )
and REAR_INITRD_COMPRESSION="lzma"
and mention that on POWER by default
FIRMWARE_FILES=( 'no' ) is used.