Docs: Add note about support for object spread (fixes #11136) (#11395)

docs/rules/prefer-object-spread.md

@@ -2,7 +2,7 @@
 
 When Object.assign is called using an object literal as the first argument, this rule requires using the object spread syntax instead. This rule also warns on cases where an `Object.assign` call is made using a single argument that is an object literal, in this case, the `Object.assign` call is not needed.
 
-Object spread is a declarative alternative which may perform better than the more dynamic, imperative `Object.assign`.
+Introduced in ES2018, object spread is a declarative alternative which may perform better than the more dynamic, imperative `Object.assign`.
 
 ## Rule Details
 
@@ -46,4 +46,4 @@ Object.assign(foo, { ...baz });
 
 ## When Not To Use It
 
-When object spread is not available in your codebase.
+This rule should not be used unless ES2018 is supported in your codebase.