docs: update shell examples to show JSON output format

weed-shell.md

@@ -243,22 +243,17 @@ The shell provides a set of composable commands for managing S3 users, policies,
 ```
 > s3.policy -put -name=photos-rw -file=photos-policy.json
 > s3.user.create -name alice
-Created user "alice"
-Access Key: AKID...
-Secret Key: ...
-
+{"name":"alice","access_key":"AKID..."}
 > s3.policy.attach -policy photos-rw -user alice
-Attached policy "photos-rw" to user "alice"
+{"policy":"photos-rw","user":"alice"}
 ```
 
+The secret key is printed to stderr only (not in the JSON output).
+
 Or use the one-step convenience command:
 ```
 > s3.user.provision -name alice -bucket photos -role readwrite
-Created policy "photos-alice-readwrite"
-Created user "alice" with policy "photos-alice-readwrite" attached
-
-Access Key: AKID...
-Secret Key: ...
+{"name":"alice","access_key":"AKID...","policy":"photos-alice-readwrite"}
 ```
 
 Roles: `readonly`, `readwrite`, `admin`. The bucket must be a literal name (no wildcards).