chore(crypto): Improve some docs

Co-authored-by: Denis Kasak <dkasak@termina.org.uk>

crates/matrix-sdk-crypto/src/identities/manager.rs

@@ -174,10 +174,10 @@ impl IdentityManager {
             .filter(|s| s != self.user_id().server_name());
         let successful_servers = response.device_keys.keys().map(|u| u.server_name());
 
-        // Append the new failed servers and remove any successful servers. We remove
-        // the successful servers explicitly so we keep incrementing the delay
-        // on the failed servers instead of removing them when the entry times
-        // out.
+        // Append the new failed servers and remove any successful servers. We
+        // need to explicitly remove the successful servers because the cache
+        // doesn't automatically remove entries that elapse. Instead, the effect
+        // is that elapsed servers will be retried and their delays incremented.
         self.failures.extend(failed_servers);
         self.failures.remove(successful_servers);
 

crates/matrix-sdk-crypto/src/utilities.rs

@@ -53,7 +53,7 @@ where
 /// A TTL cache where items get inactive instead of discarded.
 ///
 /// The items need to be explicitly removed from the cache. This allows us to
-/// implement a exponential backoff based TTL.
+/// implement exponential backoff based TTL.
 #[derive(Clone, Debug)]
 pub struct FailuresCache<T: Eq + Hash> {
     inner: Arc<RwLock<HashMap<T, FailuresItem>>>,
@@ -112,7 +112,7 @@ where
     /// Extend the cache with the given iterator of items.
     ///
     /// Items that are already part of the cache, whether they are expired or
-    /// not, will have their TTL extended using a exponential backoff
+    /// not, will have their TTL extended using an exponential backoff
     /// algorithm.
     pub fn extend(&self, iterator: impl Iterator<Item = T>) {
         let mut lock = self.inner.write().unwrap();