docs: add MutuallyExclusiveCallback tsdoc

Author: mikesposito

packages/keyring-controller/src/KeyringController.ts

@@ -364,6 +364,12 @@ export type ExportableKeyEncryptor = GenericEncryptor & {
   importKey: (key: string) => Promise<unknown>;
 };
 
+/**
+ * A function executed within a mutually exclusive lock, with
+ * a mutex releaser in its option bag.
+ *
+ * @param releaseLock - A function to release the lock.
+ */
 type MutuallyExclusiveCallback<T> = ({
   releaseLock,
 }: {