verbs: Document that pointer-returning verbs set errno on failure

The RETURN VALUE sections of the pointer-returning verbs stated only
that NULL is returned on failure. Where errno was mentioned it described
the paired int-returning sibling (e.g. ibv_dealloc_pd, ibv_free_dm), not
the pointer verb itself; only ibv_import_dm documented that errno is set
on the NULL return.

Document the errno contract on all of these verbs for consistency with
ibv_import_dm and with the sibling int-returning verbs: ibv_reg_mr and
friends, ibv_alloc_pd, ibv_alloc_dm, ibv_reg_dm_mr, ibv_alloc_null_mr,
ibv_create_ah, ibv_create_ah_from_wc, ibv_create_comp_channel,
ibv_create_cq, ibv_create_cq_ex, ibv_create_qp, ibv_create_srq,
ibv_open_device, ibv_import_device, ibv_import_mr and ibv_import_pd. The
ibv_reg_mr path has set errno since commit f9e127a4bffa ("verbs: Set
missing errno in ibv_cmd_reg_mr").

Signed-off-by: Christian Häggström <christian.haggstrom@sambanovasystems.com>