Update the documentation of the subscription id

Match the changes introduced in  https://github.com/nostr-protocol/nips/pull/299

lib/nostr/client.rb

@@ -74,7 +74,8 @@ module Nostr
     # @example Subscribing to all events created after a certain time
     #   subscription = client.subscribe(filter: Nostr::Filter.new(since: 1230981305))
     #
-    # @param subscription_id [String] The subscription id. A random string.
+    # @param subscription_id [String] The subscription id. An arbitrary, non-empty string of max length 64
+    #   chars used to represent a subscription.
     # @param filter [Filter] A set of attributes that represent the events that the client is interested in.
     #
     # @return [Subscription] The subscription object

lib/nostr/subscription.rb

@@ -5,7 +5,7 @@ require 'securerandom'
 module Nostr
   # A subscription the result of a request to receive events from a relay
   class Subscription
-    # A random string that should be used to represent a subscription
+    # An arbitrary, non-empty string of max length 64 chars used to represent a subscription
     #
     # @api public
     #
@@ -41,7 +41,7 @@ module Nostr
     # @example Subscribing to all events created after a certain time
     #   subscription = Nostr::Subscription.new(filter: Nostr::Filter.new(since: 1230981305))
     #
-    # @param id [String] A random string that should be used to represent a subscription
+    # @param id [String] An arbitrary, non-empty string of max length 64 chars used to represent a subscription
     # @param filter [Filter] An object that determines what events will be sent in that subscription
     #
     def initialize(filter:, id: SecureRandom.hex)