docs: explain cabal update repository index state

[KeuqS]

Summary

• Document the REPO[,INDEX-STATE] argument accepted by cabal update.
• Explain Unix timestamp, ISO 8601 UTC timestamp, and HEAD forms.
• Clarify that the supplied index state selects the global package index revision and is distinct from a project-local index-state.

Verification

• Compared the user guide with doc/cmd-v2-help/update.txt to keep the syntax and examples aligned.
• Documentation-only change; no runtime code or tests are affected.

Checklist

• This PR does not modify behaviour or interface.
• The documentation has been updated.

Fixes #12447

[KeuqS]

Updated the branch with current master in commit cd0052e so the PR is no longer out of date. The documentation change remains a single-file, 13-line addition and git diff --check passes; the newly triggered documentation checks are now running.

[Mikolaj]

Thank you.

[ffaf1]

Many thanks.