Mark the synonym token filter as updateable and provide a better example

[damienalexandre]

Reading https://www.elastic.co/blog/boosting-the-power-of-elasticsearch-with-synonyms - we quickly see Emoji Search can benefit from the new POST /synonym_test/_reload_search_analyzers API.

Index-time synonyms have several disadvantages:

• The index might get bigger, because all synonyms must be indexed.
• Search scoring, which relies on term statistics, might suffer because synonyms are also counted, and the statistics for less common words become skewed.
• Synonym rules can’t be changed for existing documents without reindexing.

...

Using synonyms in search-time analyzers on the other hand doesn’t have many of the above mentioned problems:

• The index size is unaffected.
• The term statistics in the corpus stay the same.
• Changes in the synonym rules don’t require reindexing of documents.

And:

Starting with Elasticsearch 7.3, this reopening of indices in order to see changes in synonym files is no longer needed.

We must:

• provide a new "search time" config
• write a better documentation for updating synonyms in production

[damienalexandre]

Using the synonym as a graph filter and at search time could also be better.

https://www.adelean.com/blog/20210421_synonym_graph_in_elasticsearch/