doc(schema-diff): Add documentation on schema-diff script

This also will include a link to this documentation when leaving a
comment on a pull request.

.github/workflows/schema-diff.yml

@@ -59,6 +59,9 @@ jobs:
           cat << EOF > github-comment.txt
           $gh_comment_prefix \`${GITHUB_BASE_REF}\` and \`${GITHUB_HEAD_REF}\` @ ${{ github.event.pull_request.head.sha }}
 
+          To understand how these diffs are generated and some limitations see the
+          [documentation](https://github.com/hashicorp/boundary/blob/main/scripts/schema-diff.sh) of the script.
+
           #### Functions
           $(getdiff .schema-diff/funcs.diff)