ESQL: Document the cast operator (::) (#107871)

This documents the cast operator, `::`.

docs/reference/esql/functions/cast.asciidoc

@@ -0,0 +1,17 @@
+[discete]
+[[esql-cast-operator]]
+==== `Cast (::)`
+
+// tag::body[]
+The `::` operator provides a convenient alternative syntax to the TO_<type>
+<<esql-type-conversion-functions,conversion functions>>.
+
+[source.merge.styled,esql]
+----
+include::{esql-specs}/convert.csv-spec[tag=docsCastOperator]
+----
+[%header.monospaced.styled,format=dsv,separator=|]
+|===
+include::{esql-specs}/convert.csv-spec[tag=docsCastOperator-result]
+|===
+// end::body[]

docs/reference/esql/functions/operators.asciidoc

@@ -12,6 +12,7 @@ Boolean operators for comparing against one or multiple expressions.
 * <<esql-unary-operators>>
 * <<esql-logical-operators>>
 * <<esql-predicates>>
+* <<esql-cast-operator>>
 * <<esql-cidr_match>>
 * <<esql-in-operator>>
 * <<esql-like-operator>>
@@ -22,6 +23,7 @@ include::binary.asciidoc[]
 include::unary.asciidoc[]
 include::logical.asciidoc[]
 include::predicates.asciidoc[]
+include::cast.asciidoc[]
 include::cidr_match.asciidoc[]
 include::in.asciidoc[]
 include::like.asciidoc[]

x-pack/plugin/esql/qa/testFixtures/src/main/resources/convert.csv-spec

@@ -167,3 +167,16 @@ required_feature: esql.casting_operator
 10002          |true                 |false          |2.08
 10004          |true                 |false          |1.78
 ;
+
+docsCastOperator
+required_feature: esql.casting_operator
+//tag::docsCastOperator[]
+ROW ver = CONCAT(("0"::INT + 1)::STRING, ".2.3")::VERSION
+//end::docsCastOperator[]
+;
+
+//tag::docsCastOperator-result[]
+ver:version
+1.2.3
+//end::docsCastOperator-result[]
+;