Improvements to docs around multiplexer and synonyms (#41645)

This commit fixes a multiplexer doc error concerning synonyms, and adds
suggestions on how to combine the two filters.

docs/reference/analysis/tokenfilters/multiplexer-tokenfilter.asciidoc

@@ -116,9 +116,8 @@ And it'd respond:
 duplicate of this token it has been removed from the token stream
 
 NOTE: The synonym and synonym_graph filters use their preceding analysis chain to
-parse and analyse their synonym lists, and ignore any token filters in the chain
-that produce multiple tokens at the same position.  This means that any filters
-within the multiplexer will be ignored for the purpose of synonyms.  If you want to
-use filters contained within the multiplexer for parsing synonyms (for example, to
-apply stemming to the synonym lists), then you should append the synonym filter
-to the relevant multiplexer filter list.
+parse and analyse their synonym lists, and will throw an exception if that chain
+contains token filters that produce multiple tokens at the same position.
+If you want to apply synonyms to a token stream containing a multiplexer, then you
+should append the synonym filter to each relevant multiplexer filter list, rather than
+placing it after the multiplexer in the main token chain definition.

docs/reference/analysis/tokenfilters/synonym-graph-tokenfilter.asciidoc

@@ -188,6 +188,10 @@ parsing synonyms, e.g. `asciifolding` will only produce the folded version of th
 token.  Others, e.g. `multiplexer`, `word_delimiter_graph` or `ngram` will throw an
 error.
 
+If you need to build analyzers that include both multi-token filters and synonym
+filters, consider using the <<analysis-multiplexer-tokenfilter,multiplexer>> filter,
+with the multi-token filters in one branch and the synonym filter in the other.
+
 WARNING: The synonym rules should not contain words that are removed by
 a filter that appears after in the chain (a `stop` filter for instance).
 Removing a term from a synonym rule breaks the matching at query time.

docs/reference/analysis/tokenfilters/synonym-tokenfilter.asciidoc

@@ -177,3 +177,7 @@ multiple versions of a token may choose which version of the token to emit when
 parsing synonyms, e.g. `asciifolding` will only produce the folded version of the
 token.  Others, e.g. `multiplexer`, `word_delimiter_graph` or `ngram` will throw an
 error.
+
+If you need to build analyzers that include both multi-token filters and synonym
+filters, consider using the <<analysis-multiplexer-tokenfilter,multiplexer>> filter,
+with the multi-token filters in one branch and the synonym filter in the other.