update examples and add documentation

03-builder-pattern/src/main.rs

@@ -41,9 +41,9 @@ impl UnlockedCredentialManager {
 }
 
 fn main() {
-    let pw = LockedCredentialManager::new();
-    let mut pw = pw.unlock("pssst");
-    pw.set("foo", "bar");
-    let val = pw.get("foo");
-    pw.lock();
+    let pw_locked = LockedCredentialManager::new();
+    let mut pw_unlocked = pw_locked.unlock("pssst");
+    pw_unlocked.set("foo", "bar");
+    let val = pw_unlocked.get("foo");
+    let pw_locked = pw_unlocked.lock();
 }