docs: Mention field mapping issue with ElasticsearchWriter

[martialblog]

I think it would be good to put a warning label on the enable_send_perfdata feature of the ElasticsearchWriter, just so that users are aware of its implications.

See #10511 and #6805

Wasn't sure which "note" syntax is right for this, found these three in the docs.

Note: xxx

> **Note**
>
> xxx

!!! note

    xxx

[oxzi]

Thanks a lot for improving the docs!

Because this was a small change, there are of course lots of comments 🙃

[martialblog]

These changes would be made obsolete by: #10518

[oxzi]

Thanks!