Deprecate :title: direcive

Author: st0012

lib/rdoc/markup/pre_process.rb

@@ -223,6 +223,13 @@ def handle_directive prefix, directive, param, code_object = nil,
     when 'title' then
       @options.default_title = param if @options.respond_to? :default_title=
 
+      warn <<~MSG
+        The :title: directive is deprecated and will be removed in RDoc 7.
+
+        Use `--title=#{param}` to specify the title displayed.
+        If you use `RDoc::Task`, use `rdoc.title = "#{param}"` instead.
+      MSG
+
       blankline
     when 'yield', 'yields' then
       return blankline unless code_object

lib/rdoc/parser/c.rb

@@ -1113,6 +1113,13 @@ def look_for_directives_in context, comment
         ''
       when 'title' then
         @options.default_title = param if @options.respond_to? :default_title=
+
+        warn <<~MSG
+          The :title: directive is deprecated and will be removed in RDoc 7.
+
+          Use `--title=#{param}` to specify the title displayed.
+          If you use `RDoc::Task`, use `rdoc.title = "#{param}"` instead.
+        MSG
         ''
       end
     end