[Serverless]: Document the alternative to externalized synonyms file in Elasticsearch Serverless

[ppf2]

Serverless Docs

Welcome to Elastic Serverless

Description

https://www.elastic.co/guide/en/serverless/current/elasticsearch-differences.html#elasticsearch-differences-serverless-feature-unavailable

We mention that custom plugins and bundles are not supported on Serverless.

Since customer bundles are often used for synonyms, it will be helpful to include the workaround for specific use cases of bundles as part of our limitations/differences documentation.

For example, for synonyms, the alternative to uploading externalized synonym files is to use the new https://www.elastic.co/docs/api/doc/elasticsearch-serverless/operation/operation-synonyms-get-synonym API to manage synonyms.

Resources and additional context

https://www.elastic.co/guide/en/serverless/current/elasticsearch-differences.html#elasticsearch-differences-serverless-feature-unavailable

[shainaraskas]

@leemthompo I think https://www.elastic.co/docs/solutions/search/full-text/search-with-synonyms needs some clarification re: what works in serverless and what works in stack here.

related to the page, but not to serverless: https://github.com/elastic/docs-projects/issues/450 - the instructions linked from this doc are for cloud and ece only