Remove the 'default' attribute, make all rules have rule IDs & make server default rules IDs start with '.'.

drafts/push_csapi.rst

@@ -100,9 +100,8 @@ rules of the same kind on different devices.
 
 A home server may also have server default rules of each kind and in each scope.
 Server default rules are lower priority than user-defined rules in each scope.
-Server defined rules do not have a rule_id except when it is necessary to derive
-the function of the rule (ie. in room and sender rules). Server default rules
-have an attribute, "default" set to true.
+Server default rules (and only server default rules) begin with a dot ('.')
+character.
 
 In addition, all rules may be enabled or disabled. Disabled rules never match.