linux/pipe_fs_i.h: fix kernel-doc warnings after @wait was split

Fix kernel-doc warnings in struct pipe_inode_info after @wait was
split into @rd_wait and @wr_wait.

  include/linux/pipe_fs_i.h:66: warning: Function parameter or member 'rd_wait' not described in 'pipe_inode_info'
  include/linux/pipe_fs_i.h:66: warning: Function parameter or member 'wr_wait' not described in 'pipe_inode_info'

Fixes: 0ddad21d ("pipe: use exclusive waits when reading or writing")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>