Add comments to async query handlers

[ykmr1224]

Description

Added comments to async query handlers to make the responsibility of each handler clearer.

Issues Resolved

N/A

Check List

• [x] New functionality includes testing.
  • [x] All tests pass, including unit test, integration test and doctest
• [x] New functionality has been documented.
  • [x] New functionality has javadoc added
  • [x] New functionality has user manual doc added
• [x] Commits are signed per the DCO using --signoff

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[ykmr1224]

Thanks! While you're at it, it might also be nice to link to related classes, to make it slightly easier to navigate if someone e.g. lands on IndexDMLResultStorageService without having seen IndexDMLResultStorage.

That's a good point. I'm wondering if we need those links in the comments considering IDE or Github provide code navigation feature. I think the link would be beneficial when we want to mention some related class which is not obviously referred in the code.

[opensearch-trigger-bot[bot]]

The backport to 2.x 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/sql/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/sql/backport-2.x
# Create a new branch
git switch --create backport/backport-2657-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 05a2f66a0af2baab747aa0afd437441dbd5efc67
# Push it to GitHub
git push --set-upstream origin backport/backport-2657-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/sql/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-2657-to-2.x.