ux: improve the usage doc of --image-spec

[wangxiaoxuan273]

What this PR does / why we need it:

auto option for --image-spec is removed.

current display of --image-spec help info:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #1061

Please check the following list:

• [ ] Does the affected code have corresponding tests, e.g. unit test, E2E test?
• [x] Does this change require a documentation update?
• [ ] Does this introduce breaking changes that would require an announcement or bumping the major version?
• [ ] Do all new files have an appropriate license header?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.40%. Comparing base (1494475) to head (6292d7e).

:exclamation: Current head 6292d7e differs from pull request most recent head f46b46c. Consider uploading reports for the commit f46b46c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1334 +/- ## ========================================== - Coverage 84.41% 84.40% -0.01% ========================================== Files 97 97 Lines 3541 3540 -1 ========================================== - Hits 2989 2988 -1 Misses 329 329 Partials 223 223 ```

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