Update searchable snapshot documentation to be more correct

[adaisley]

This is more in-line with what is defined in the example Opensearch docker-compose.
I tried the current approach and it straight-up did not work in its current form.

Description

Give correct instructions in the Searchable Snapshots documentation page.

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].
None

Checklist

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[adaisley]

I will say that the documentation surrounding the Searchable Snapshots feature and notably the Search node role is extremely limited right now. You cannot just 'add' that search node declaration to existing nodes as it seems to require the data/ folder for that node needs to not contain any data (and running the cleanup tool in Docker didn't seem to do anything only complain that the cluster isn't running, when I tried to run it when the cluster was up it said it needed to not be running...)

Tried the original method that the page describes i.e. add - node.roles: [ search ] to the docker-compose file but it complained about using : instead of = to add this. Then as an array I was getting weird behaviour so took that out and can observe a correctly-functioning test environment Search node that successfully pulls data from a remote snapshot repository.

[opensearch-trigger-bot]

The backport to 1.3 failed:

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

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-1.3 1.3
# Navigate to the new working tree
pushd ../.worktrees/backport-1.3
# Create a new branch
git switch --create backport/backport-4203-to-1.3
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 6bf867a200cfe71f1203c7350fd64f2b56d20085
# Push it to GitHub
git push --set-upstream origin backport/backport-4203-to-1.3
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-1.3

Then, create a pull request where the base branch is 1.3 and the compare/head branch is backport/backport-4203-to-1.3.

[opensearch-trigger-bot]

The backport to 2.8 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-2.8 2.8
# Navigate to the new working tree
pushd ../.worktrees/backport-2.8
# Create a new branch
git switch --create backport/backport-4203-to-2.8
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 6bf867a200cfe71f1203c7350fd64f2b56d20085
# Push it to GitHub
git push --set-upstream origin backport/backport-4203-to-2.8
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.8

Then, create a pull request where the base branch is 2.8 and the compare/head branch is backport/backport-4203-to-2.8.