[wip][rfc] Reference level explanation

rfcs/0000-limitador-multithread-inmemory.md

@@ -130,14 +130,28 @@ These scenarios will be explained in detail in the [Reference-level explanation]
 # Reference-level explanation
 [reference-level-explanation]: #reference-level-explanation
 
-This is the technical portion of the RFC. Explain the design in sufficient detail that:
-
-- Its interaction with other features is clear.
-- It is reasonably clear how the feature would be implemented.
-- How error would be reported to the users.
-- Corner cases are dissected by example.
+This guide is going to be focused on the _InMemory_ storage option from Limitador, but the same approach could be applied to other storage options.
+We will be using as reference a simplified version of the method signature `check_and_update` from the `Storage impl`
+block  [from the current Limitador crate](https://github.com/Kuadrant/limitador/blob/main/limitador/src/storage/mod.rs#L126-L134)
+
+```rust
+pub fn check_and_update(
+        &self,
+        counters: &mut Vec<Counter>,
+        delta: i64,
+    ) -> Result<Authorization, StorageErr> {
+        self.counters
+            .check_and_update(counters, delta)
+    }
+```
 
-The section should return to the examples given in the previous section, and explain more fully how the detailed proposal makes those examples work.
+In brief, the `check_and_update` method is called by the `Limitador` service when a request is received. It receives a list of
+`Counter` objects, which represent the limits that the request is trying to access, and a `delta` value, which represents
+the amount of requests that the service is trying to authorize. It will iterate over the list of `Counter` objects and
+will check if the request can be authorized or not. If the request can be authorized, the `Counter` object will be updated
+with the new value. If the request cannot be authorized, the `Counter` object will not be updated and the method will
+return an `Authorization` object with the `Authorization::Limited` value, otherwise the method will return an
+`Authorization` object with the `Authorization::Ok` value.
 
 # Drawbacks
 [drawbacks]: #drawbacks