ext4: clairfy the rules for modifying extents

Add a comment at the beginning of extents_status.c to clarify the rules
for loading, mapping, modifying, and removing extents and blocks.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250423085257.122685-10-yi.zhang@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>