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?
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

❗ 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

@@            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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[qweeah]

LGTM