doc: update layer.md

Author: tyru

_docs/layer.md

@@ -4,11 +4,9 @@
 The volt commands like `volt get` which may modify lock.json, config.toml,
 filesystem, are executed in several steps:
 
-1. (UI layer): pass subcommand arguments, lock.json & config.toml structure
-   to Gateway layer
-2. (Gateway layer): Create an AST according to given information
-    * This layer cannot touch filesystem, because it makes unit testing difficult
-3. (Usecase layer): Execute the AST. This note mainly describes this layer's design
+1. (UI layer): Passes subcommand arguments, lock.json & config.toml structure to Gateway layer
+2. (Gateway layer): Invokes usecase(s). This layer cannot touch filesystem, do network requests, because it makes unit testing difficult
+3. (Usecase layer): Modify files, do network requests, and other business logic
 
 Below is the dependency graph: