doc: improve doc for RPCArg::Optional::OMITTED

Author: fanquake

src/rpc/util.h

@@ -154,14 +154,11 @@ struct RPCArg {
         /** Required arg */
         NO,
         /**
-         * The arg is optional for one of two reasons:
-         *
-         * Optional arg that is a named argument and has a default value of
-         * `null`.
-         *
-         * Optional argument with default value omitted because they are
-         * implicitly clear. That is, elements in an array may not
-         * exist by default.
+         * Optional argument for which the default value is omitted from
+         * help text for one of two reasons:
+         * - It's a named argument and has a default value of `null`.
+         * - Its default value is implicitly clear. That is, elements in an
+         *    array may not exist by default.
          * When possible, the default value should be specified.
          */
         OMITTED,