feat: update kusion cli descriptions

[liu-hm19]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR updates the --help descriptions of the Kusion CLI.

Which issue(s) this PR fixes:

Fixes #811

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Additional documentation e.g., design docs, usage docs, etc.:

[coveralls]

Pull Request Test Coverage Report for Build 8981107032

Details

• 5 of 39 (12.82%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 54.805%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/cmd/mod/mod.go	0	1	0.0%
pkg/cmd/mod/mod_push.go	0	2	0.0%
pkg/cmd/apply/apply.go	2	5	40.0%
pkg/cmd/generate/generate.go	0	3	0.0%
pkg/cmd/preview/preview.go	3	8	37.5%
pkg/cmd/cmd.go	0	20	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/cmd/preview/preview.go	1	42.28%

Totals
Change from base Build 8891007653:	0.04%
Covered Lines:	8834
Relevant Lines:	16119

---

💛 - Coveralls

[SparkYuan]

lgtm