[ES|QL] Document return value for locate in case substring is not found

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

@@ -284,7 +284,7 @@ ip_prefix     |Truncates an IP to a given prefix length.
 least         |Returns the minimum value from multiple columns. This is similar to <<esql-mv_min>> except it is intended to run on multiple columns at once.
 left          |Returns the substring that extracts 'length' chars from 'string' starting from the left.
 length        |Returns the character length of a string.
-locate        |Returns an integer that indicates the position of a keyword substring within another string.
+locate        |Returns an integer that indicates the position of a keyword substring within another string. Returns `0` if the substring cannot be found. Note that string positions start from `1`.
 log           |Returns the logarithm of a value to a base. The input can be any numeric value, the return value is always a double.  Logs of zero, negative numbers, and base of one return `null` as well as a warning.
 log10         |Returns the logarithm of a value to base 10. The input can be any numeric value, the return value is always a double.  Logs of 0 and negative numbers return `null` as well as a warning.
 ltrim         |Removes leading whitespaces from a string.

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/string/Locate.java

@@ -46,11 +46,10 @@ public class Locate extends EsqlScalarFunction implements OptionalArgument {
     private final Expression substr;
     private final Expression start;
 
-    @FunctionInfo(
-        returnType = "integer",
-        description = "Returns an integer that indicates the position of a keyword substring within another string.",
-        examples = @Example(file = "string", tag = "locate")
-    )
+    @FunctionInfo(returnType = "integer", description = """
+        Returns an integer that indicates the position of a keyword substring within another string.
+        Returns `0` if the substring cannot be found.
+        Note that string positions start from `1`.""", examples = @Example(file = "string", tag = "locate"))
     public Locate(
         Source source,
         @Param(name = "string", type = { "keyword", "text" }, description = "An input string") Expression str,