Docs: change when not to use object spread (#10621)

Object spread is not just syntax sugar. It actually performs better in browsers. The "When Not To Use It" section should therefore give a more nuanced opinion on when Object.assign is desireable.

docs/rules/prefer-object-spread.md

@@ -2,6 +2,8 @@
 
 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`.
+
 ## Rule Details
 
 Examples of **incorrect** code for this rule:
@@ -44,4 +46,4 @@ Object.assign(foo, { ...baz });
 
 ## When Not To Use It
 
-When you don't care about syntactic sugar added by the object spread property.
+When object spread is not available in your codebase.