Update default.conf

Added comment to default.conf that tells when
KEEP_BUILD_DIR is automatically set to true
cf. #2121

usr/share/rear/conf/default.conf

@@ -127,7 +127,9 @@ OS_VERSION=none
 
 # Keep the build area after we are done (BOOL).
 # Useful to inspect the ReaR recovery system content in $TMPDIR/rear.XXXXXXXXXXXXXXX/rootfs/
-# directly without the need to extract it from the initramfs/initrd in the ISO image:
+# directly without the need to extract it from the initramfs/initrd in the ISO image.
+# KEEP_BUILD_DIR is automatically set to true in debug mode (-d) and in debugscript mode (-D)
+# and by build/default/990_verify_rootfs.sh when it detects errors in the ReaR recovery system.
 KEEP_BUILD_DIR=""
 
 # No default workflows. This variable is filled in where the workflows are defined