fs: Clarify FS_CASEFOLD_FL semantics in UAPI header

fs: Clarify FS_CASEFOLD_FL semantics in UAPI header

The existing one-liner "Folder is case insensitive" leaves the
impression that FS_CASEFOLD_FL is reserved for directories.
That impression is wrong: filesystems that derive
case-insensitivity from mount or volume state report the bit on
non-directory inodes via i_op->fileattr_get, so userspace
inspecting FS_IOC_GETFLAGS can see it on any inode type.

Replace the one-liner with a block comment that names directories
as the typical case, records that non-directory inodes may also
report the bit, and notes FS_XFLAG_CASEFOLD as the read-only
companion exposed through FS_IOC_FSGETXATTR.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260507-case-sensitivity-v14-0-e62cc8200435@oracle.com?part=3
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20260515153515.362266-5-cel@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>