Improve linkage check script documentation for git references

[zhan7236]

The usage documentation in beam-linkage-check.sh showed origin/master as an example, which incorrectly suggested that users must have a remote called "origin". The script actually accepts any valid git reference.

Changes:

• Use generic <baseline ref> placeholder instead of origin/master
• Add examples of valid git references:
  • Remote branches: origin/master, upstream/main
  • Local branches: master, my-feature-branch
  • Commit SHAs: abc123def
  • Tags: v2.50.0

Fixes #20650

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[gemini-code-assist]

Summary of Changes

Hello @zhan7236, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves the documentation for the beam-linkage-check.sh script by making its usage instructions more generic and providing clear examples of the various git references it supports. This enhancement ensures that users can more easily understand and utilize the script with different types of git references, removing ambiguity from the previous example.

Highlights

• Documentation Clarity: The usage example in beam-linkage-check.sh has been updated from a specific origin/master to a generic <baseline ref> placeholder, clarifying that the script accepts any valid git reference, not just a remote named 'origin'.
• Expanded Git Reference Examples: Comprehensive examples of valid git references have been added to the script's documentation, including remote branches (e.g., origin/master), local branches (e.g., master), commit SHAs (e.g., abc123def), and tags (e.g., v2.50.0), to improve user understanding and flexibility.

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[github-actions]

Assigning reviewers:

R: @chamikaramj for label java.
R: @liferoad for label build.

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[github-actions]

Reminder, please take a look at this pr: @chamikaramj @liferoad

[github-actions]

Assigning new set of reviewers because Pr has gone too long without review. If you would like to opt out of this review, comment assign to next reviewer:

R: @kennknowles for label java.
R: @Abacn for label build.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

[kennknowles]

LGTM. In fact, I don't have any remote named origin because I like more specific names, so I agree very much :-)