Revert "Remove docs warning that synthetic source is in es|ql is experimental…" (#130170)

Reverts #129952

Author: martijnvg

docs/reference/esql/esql-limitations.asciidoc

@@ -99,6 +99,9 @@ unless they're <<esql-multi-index-union-types, explicitly converted to a single
 {esql} does not support configurations where the
 <<mapping-source-field,_source field>> is <<disable-source-field,disabled>>.
 
+experimental:[] {esql}'s support for <<synthetic-source,synthetic `_source`>>
+is currently experimental.
+
 [discrete]
 [[esql-limitations-full-text-search]]
 === Full-text search