Add comments to a configuration example file

Users can create their own config files by copying the configuration
examples, so we add comments to make the settings easier for users.

yorkie/config.sample.yml

@@ -1,25 +1,72 @@
+# RPC is the configuration for the RPC server.
 RPC:
-  # comment 
+  # Port to listen on for RPC connections (default: 11101).
   Port: 11101
+
+  # CertFile is the file containing the TLS certificate.
   CertFile: ""
+
+  # KeyFile is the file containing the TLS private key.
   KeyFile: ""
 
 Metrics:
-  # comment
+  # Port to listen on for Prometheus metrics `/metrics` (default: 11102).
   Port: 11102
 
+# Backend is the configuration for the backend of Yorkie.
+Backend:
+  # SnapshotThreshold is the threshold that determines if changes should be
+  # sent with snapshot when the number of changes is greater than this value.
+  SnapshotThreshold: 500
+
+  # SnapshotInterval is the interval of changes to create a snapshot.
+  SnapshotInterval: 100
+
+  # AuthWebhookURL is the URL to send authorization requests to.
+  AuthWebhookURL: ""
+
+  # AuthWebhookMethods is the list of methods to use for authorization.
+  AuthWebhookMethods: []
+
+  # AuthWebhookMaxRetries is the max count that retries the authorization webhook.
+  AuthWebhookMaxRetries: 10
+
+  # AuthWebhookMaxWaitInterval is the max interval that waits before retrying the authorization webhook.
+  AuthWebhookMaxWaitInterval: "3s"
+
+  # AuthWebhookCacheAuthTTL is the TTL value to set when caching the authorized result.
+  AuthWebhookCacheAuthTTL: "10s"
+
+  # AuthWebhookCacheUnauthTTL is the TTL value to set when caching the unauthorized result.
+  AuthWebhookCacheUnauthTTL: "10s"
+
+# Mongo is the MongoDB configuration.
 Mongo:
-  # comment
-  ConnectionTimeout: "5"
+  # ConnectionTimeout is the timeout for connecting to MongoDB.
+  ConnectionTimeout: "5s"
+
+  # ConnectionURI is the URI to connect to MongoDB.
   ConnectionURI: "mongodb://localhost:27017"
+
+  # YorkieDatabase is the name of the Yorkie database.
   YorkieDatabase: "yorkie-meta"
-  PingTimeout: "5"
 
+  # PingTimeout is the timeout for pinging MongoDB.
+  PingTimeout: "5s"
+
+# ETCD is the configuration for the etcd client.
 ETCD:
-  Endpoints: [
-    "localhost:2379"
-    ]
+  # Endpoints is the list of endpoints to connect to for etcd.
+  Endpoints: ["localhost:2379"]
 
-Backend:
-  SnapshotThreshold: 500
-  SnapshotInterval: 100
+  # DialTimeoutSec is the timeout for connecting to etcd.
+  DialTimeoutSec: "5"
+
+  # Username is the username to use for etcd.
+  Username: ""
+
+  # Password is the password to use for etcd.
+  Password: ""
+
+  # LockLeaseTimeSec is the lease time for locks.
+  LockLeaseTimeSec: "30"