Migrate ABC size doc from Markdown to AsciiDoc for Antora

Follow up to rubocop#10076.

lib/rubocop/cop/metrics/abc_size.rb

@@ -8,6 +8,12 @@ module Metrics
       # (method calls), and conditions. See http://c2.com/cgi/wiki?AbcMetric
       # and https://en.wikipedia.org/wiki/ABC_Software_Metric.
       #
+      # Interpreting ABC size:
+      #
+      # * <= 17 satisfactory
+      # * 18..30 unsatisfactory
+      # * > 30 dangerous
+      #
       # You can have repeated "attributes" calls count as a single "branch".
       # For this purpose, attributes are any method with no argument; no attempt
       # is meant to distinguish actual `attr_reader` from other methods.
@@ -28,12 +34,6 @@ module Metrics
       #    end
       #
       # This cop also takes into account `IgnoredMethods` (defaults to `[]`)
-      #
-      # ## Interpreting ABCSize
-      #
-      # - <= 17 satisfactory
-      # - 18..30 unsatisfactory
-      # - > 30 dangerous
       class AbcSize < Base
         include MethodComplexity