docs: erofs: refine the on-disk overview

The existing on-disk summary mainly describes inode and shared xattr
metadata views, leaving the overall layout and design rationale unclear.

Expand the summary and refine the terminology and wording while keeping
the section a high-level overview.

Assisted-by: chatgpt:gpt-6.1-sol
Reviewed-by: Chengyu Zhu <hudsonzhu@tencent.com>
Signed-off-by: Gao Xiang <xiang.gao@docker.com>