check_format.sh: fix JSON-RPC doc checks

The check needs to fail if documentation is missing
for any RPC. But the way the check was written, the
rc would get set to 0 everytime it found docs for
an RPC. This means it would only return failure if
the last RPC that was checked did not have
documentation.

While here, add a message telling the user that
JSON-RPC docs are being checked, to be consistent
with other checks in this script.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ib2ff0f0432ce3e2853526223c0ad00176d84a78c
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9391


Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Jacek Kalwas <jacek.kalwas@intel.com>
Reviewed-by: Dong Yi <dongx.yi@intel.com>
Reviewed-by: Xiaodong Liu <xiaodong.liu@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>