Clarify read-only index in docs

[rubpa]

Updated force-merge and freeze index API pages to clarify read-only indices.

[elasticmachine]

Pinging @elastic/es-core-features

[elasticmachine]

Pinging @elastic/es-docs

[gwbrown]

Thanks for your PR @rubpa. Two things:

1. Could you sign our contributor license agreement? This is necessary before we'll be able to merge your changes.
2. I'm not sure the change to the force merge documentation is the right change to make because freezing an index is not the only way to make it read-only - for example, setting index.blocks.write will also make an index read-only.

[rubpa]

Hi @gwbrown! While trying to manage space used by indices and finding all the new options in Kibana, I came across read-only used in the documentation but could not find any page to make an index read-only. After some digging, I found that freeze does it. Then the nice edit button gave me the thought of submitting this doc improvement as a PR.

So, it's not clear how to make read-only indices in the current documentation and I request you to do the most appropriate updates.

I saw the CLA while submitting the PR - still figuring out if I can sign it. I submitted this improvement in my personal capacity but I use elastic stack at work.

[jrodewig]

Closing as the CLA is not signed, and the force merge changes are not clear.

However, I have opened #50092 to better clarify that frozen indices are read-only.

Thanks for raising this @rubpa!