IB/Verbs: Improve docs for rdma-helpers

Increase the level of documentation for the rdma_cap_* helpers
introduced by Michael Wang <yun.wang@profitbricks.com>.

This patch is loosely based on a patch Michael wrote to enhance the
documentation of these functions, but has been significantly modified
in terms of verbiage.  In addition, the comments were moved from a kernel
Documentation/infiniband/ file to being inline in the header file itself
for the functions in question.  Finally, the documentation was formated
in proper kdoc format.

Signed-off-by: Michael Wang <yun.wang@profitbricks.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>