LU-8998 docs: man pages for composite layout APIs

LU-8998 docs: man pages for composite layout APIs

Added man pages for the new layout component interfaces:

llapi_layout_comp_add(3)
llapi_layout_comp_del(3)
llapi_layout_comp_extent_get(3)
llapi_layout_comp_extent_set(3)
llapi_layout_comp_flags_get(3)
llapi_layout_comp_flags_set(3)
llapi_layout_comp_flags_clear(3)
llapi_layout_comp_id_get(3)
llapi_layout_comp_move(3)
llapi_layout_comp_move_at(3)
llapi_layout_file_comp_add(3)
llapi_layout_file_comp_del(3)

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I8fd466b9cb0352bb7812f0b748816801f20f5e71
Reviewed-on: https://review.whamcloud.com/25740
Tested-by: Jenkins
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>