chore: update help doc with OCI layout support

[qweeah]

What this PR does / why we need it:
This PR mentioned OCI layout support in related command help doc.

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 #904

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?

[qweeah]

@FeynmanZhou Can you help review the wording in the updated help doc, thanks

[codecov-commenter]

Codecov Report

Merging #997 (04f8eb2) into main (f9aec91) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main     #997   +/-   ##
=======================================
  Coverage   81.08%   81.08%           
=======================================
  Files          53       53           
  Lines        2792     2792           
=======================================
  Hits         2264     2264           
  Misses        358      358           
  Partials      170      170           

Impacted Files	Coverage Δ
cmd/oras/root/attach.go	82.50% <100.00%> (ø)
cmd/oras/root/blob/fetch.go	68.26% <100.00%> (ø)
cmd/oras/root/blob/push.go	83.69% <100.00%> (ø)
cmd/oras/root/cp.go	86.36% <100.00%> (ø)
cmd/oras/root/discover.go	85.21% <100.00%> (ø)
cmd/oras/root/manifest/fetch.go	79.56% <100.00%> (ø)
cmd/oras/root/manifest/fetch_config.go	76.59% <100.00%> (ø)
cmd/oras/root/manifest/push.go	72.86% <100.00%> (ø)
cmd/oras/root/pull.go	73.62% <100.00%> (ø)
cmd/oras/root/push.go	73.83% <100.00%> (ø)
... and 2 more

[FeynmanZhou]

Just a tiny suggestion.

[qweeah]

Just a tiny suggestion.

Resolved. @FeynmanZhou Worth mentioning that the naming of the flag is still using OCI layout, e.g. --from-oci-layout

[TerryHowe]

/lgtm

[shizhMSFT]

LGTM