Improve linkage check script documentation for git references (#36925)

The usage documentation previously 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.

This change clarifies the documentation by:
- Using generic '<baseline ref>' placeholder instead of 'origin/master'
- Adding examples of valid git references (remote branches, local branches,
  commit SHAs, and tags)

Fixes #20650

Author: zhan7236

sdks/java/build-tools/beam-linkage-check.sh

@@ -20,7 +20,13 @@
 # one branch and another.
 
 # Usage:
-#  /bin/bash sdks/java/build-tools/beam-linkage-check.sh origin/master <your branch>
+#  /bin/bash sdks/java/build-tools/beam-linkage-check.sh <baseline ref> <proposed ref>
+#
+#  The <baseline ref> and <proposed ref> can be any valid git reference such as:
+#  - A remote branch: origin/master, upstream/main
+#  - A local branch: master, my-feature-branch
+#  - A commit SHA: abc123def
+#  - A tag: v2.50.0
 #
 #  By default, this checks the Maven artifacts listed in ARTIFACTS variable below.
 #