[DOCS] Add examples and descriptions for explain, termvector, search APIs

[lcawl]

Relates to #2482

This PR adds examples and descriptions copied from https://www.elastic.co/guide/en/elasticsearch/reference/current/search-template-api.html,[https://www.elastic.co/guide/en/elasticsearch/reference/current/render-search-template-api.html,https://www.elastic.co/guide/en/elasticsearch/reference/current/search-shards.html, https://www.elastic.co/guide/en/elasticsearch/reference/current/search-explain.html, https://www.elastic.co/guide/en/elasticsearch/reference/master/docs-multi-termvectors.html, https://www.elastic.co/guide/en/elasticsearch/reference/master/docs-termvectors.html

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
explain	🟢	15/15	15/15
render_search_template	🟢	1/1	1/1
search_shards	🟢	7/7	7/7
search_template	🟢	2/2	1/1

You can validate these APIs yourself by using the make validate target.

[szabosteve]

LGTM

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
explain	🟢	15/15	15/15
mtermvectors	🟢	10/10	10/10
render_search_template	🟢	1/1	1/1
search_shards	🟢	7/7	7/7
search_template	🟢	2/2	1/1
termvectors	🟢	8/8	8/8

You can validate these APIs yourself by using the make validate target.

[github-actions]

The backport to 8.17 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.17 8.17
# Navigate to the new working tree
cd .worktrees/backport-8.17
# Create a new branch
git switch --create backport-3614-to-8.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 52b4f94889a993ccd226e9c74ca51f773286833a
# Push it to GitHub
git push --set-upstream origin backport-3614-to-8.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.17

Then, create a pull request where the base branch is 8.17 and the compare/head branch is backport-3614-to-8.17.

[lcawl]

💚 All backports created successfully

Status	Branch	Result
✅	8.17

Questions ?

Please refer to the Backport tool documentation