include/linux/pipe_fs_i: Add htmldoc annotation for "head_tail" member

Add htmldoc annotation for the newly introduced "head_tail" member
describing it to be a union of the pipe_inode_info's @head and @tail
members.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20250305204609.5e64768e@canb.auug.org.au/


Fixes: 3d252160 ("fs/pipe: Read pipe->{head,tail} atomically outside pipe->mutex")
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>