Add PPL describe command

[YANG-DB]

update correlation comment as experimental

Description

• Update correlation documentation with experimental comment and warning
• Add ppl describe command for describing table structure

Issues Resolved

• [FEATURE]Add top PPL command #536

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[opensearch-trigger-bot]

The backport to 0.5 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/opensearch-spark/backport-0.5 0.5
# Navigate to the new working tree
pushd ../.worktrees/opensearch-spark/backport-0.5
# Create a new branch
git switch --create backport/backport-541-to-0.5
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 c2e4020d41406b019f31e24f831b1d2f2931eaff
# Push it to GitHub
git push --set-upstream origin backport/backport-541-to-0.5
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/opensearch-spark/backport-0.5

Then, create a pull request where the base branch is 0.5 and the compare/head branch is backport/backport-541-to-0.5.