Add README explination

packages/apollo-datasource-rest/README.md

@@ -70,10 +70,10 @@ class MoviesAPI extends RESTDataSource {
 ```
 
 #### `requestCacheEnabled`
-By default, `RESTDataSource` caches all outgoing GET **requests** in a separate memoized cache from the regular response cache. It does this to prevent duplicate calls that might happen in rapid succession.
-If a request is made with the same cache key (URL by default) with an HTTP method other than GET, the cached request is then cleared.
+By default, `RESTDataSource` caches all outgoing GET **requests** in a separate memoized cache from the regular response cache. It makes the assumption that all responses from HTTP GET calls are cacheable by their URL.
+If a request is made with the same cache key (URL by default) but with an HTTP method other than GET, the cached request is then cleared.
 
-If you would like to disable the GET request cache, set the `requestCacheEnabled` property to `false`.
+If you would like to disable the GET request cache, set the `requestCacheEnabled` property to `false`. You might want to do this if your API is not actually cacheable or your data changes over time.
 
 ```js title="requestCacheEnabled.js"
 class MoviesAPI extends RESTDataSource {