Mark synonym_graph as beta in the docs
We do want to keep this functionality in the future and we provide support for it.
This change is a first step towards replacing the `synonym` token filter with `synonym_graph`.
[[analysis-synonym-graph-tokenfilter]]
=== Synonym Graph Token Filter
-experimental[This functionality is marked as experimental in Lucene]
+beta[]
The `synonym_graph` token filter allows to easily handle synonyms,
including multi-word synonyms correctly during the analysis process.
