[DOCS] Document empty string boolean value as `false` (#61341)

docs/reference/mapping/types/boolean.asciidoc

@@ -10,7 +10,7 @@ strings which are interpreted as either true or false:
 [horizontal]
 False values::
 
-    `false`, `"false"`
+    `false`, `"false"`, `""` (empty string)
 
 True values::