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: 3d25216 ("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>

Author: kudureranganath

include/linux/pipe_fs_i.h

@@ -65,6 +65,7 @@ union pipe_index {
  *	@wr_wait: writer wait point in case of full pipe
  *	@head: The point of buffer production
  *	@tail: The point of buffer consumption
+ *	@head_tail: unsigned long union of @head and @tail
  *	@note_loss: The next read() should insert a data-lost message
  *	@max_usage: The maximum number of slots that may be used in the ring
  *	@ring_size: total number of buffers (should be a power of 2)