ESQL: Fix Function javadoc (#132399)

Co-authored-by: Bogdan Pintea <[email protected]>

Author: alex-spies

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/function/Function.java

@@ -17,8 +17,8 @@
 import java.util.StringJoiner;
 
 /**
- * Any SQL expression with parentheses, like {@code MAX()}, or {@code ABS()}. A
- * function is always a {@code NamedExpression}.
+ * Any ESQL function; generally this is translated into a computation to be evaluated on arguments, including scalar functions (e.g. in
+ * {@code EVAL}) or aggregation functions ({@code STATS}).
  */
 public abstract class Function extends Expression {