src.conf.5: LOADER_BIOS_TEXTONLY default change
WITH_LOADER_BIOS_TEXTONLY is now the default so document
WITHOUT_LOADER_BIOS_TEXTONLY.
Fixes: 23dee252daf2 loader: Change this BIOS tradeoff...
(cherry picked from commit 50b5a37a12d032085276b1f0ebb5f92c0cabed32)