feature: make pouchd support generating command line docs

[allencloud]

Signed-off-by: Allen Sun allensun.shl@alibaba-inc.com

Ⅰ. Describe what this PR did

make pouchd support generating command line docs。

Add a subcommand for pouchd, and this subcommand could be used to auto generate CLI docs.

Ⅱ. Does this pull request fix one issue?

none

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

no need

Ⅳ. Describe how to verify it

no need

Ⅴ. Special notes for reviews

none

[codecov]

Codecov Report

Merging #2670 into master will increase coverage by 12.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #2670       +/-   ##
==========================================
+ Coverage   56.82%   69.6%   +12.78%     
==========================================
  Files         281     281               
  Lines       18794   18794               
==========================================
+ Hits        10679   13082     +2403     
+ Misses       6879    4252     -2627     
- Partials     1236    1460      +224

Flag	Coverage Δ
#criv1alpha1test	31.64% <ø> (+0.03%)	⬆️
#criv1alpha2test	35.74% <ø> (-0.03%)	⬇️
#integrationtest	42.19% <ø> (?)
#nodee2etest	32.36% <ø> (-0.05%)	⬇️
#unittest	27.07% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
cri/ocicni/cni_manager.go	58.82% <0%> (-11.77%)	⬇️
cri/v1alpha2/cri.go	69.15% <0%> (-0.38%)	⬇️
daemon/mgr/container_utils.go	83.33% <0%> (+0.59%)	⬆️
daemon/logger/jsonfile/utils.go	71.54% <0%> (+0.81%)	⬆️
pkg/user/user.go	40% <0%> (+1.66%)	⬆️
daemon/containerio/io.go	74.75% <0%> (+1.94%)	⬆️
pkg/utils/utils.go	83.26% <0%> (+2.04%)	⬆️
ctrd/client.go	64.04% <0%> (+2.24%)	⬆️
cri/stream/runtime.go	70.23% <0%> (+2.38%)	⬆️
daemon/daemon.go	69.23% <0%> (+2.56%)	⬆️
... and 79 more

[fuweid]

LGTM