rbinstall.rb: Note about no_write [ci skip]

Since RubyGems 3.0.0 `dir_mode` option is supported, but using `File`
method to apply it, not `FileUtils`.  To reduce overwriting existing
methods (especially built-in class), and to record making directories,
keep using `no_write` method.

tool/rbinstall.rb

@@ -527,6 +527,8 @@ module RbInstall
         const_set(:FileUtils, fu::NoWrite)
         fu
       end
+      # RubyGems 3.0.0 or later supports `dir_mode`, but it uses
+      # `File` method to apply it, not `FileUtils`.
       dir_mode = options.delete(:dir_mode) if options
     end
     yield
@@ -806,6 +808,7 @@ def install_default_gem(dir, srcdir, bindir)
   install_dir = with_destdir(gem_dir)
   prepare "default gems from #{dir}", gem_dir
   RbInstall.no_write do
+    # Record making directories
     makedirs(Gem.ensure_default_gem_subdirectories(install_dir, $dir_mode).map {|d| File.join(gem_dir, d)})
   end
 
@@ -1103,6 +1106,7 @@ install?(:ext, :comm, :gem, :'bundled-gems') do
   install_dir = with_destdir(gem_dir)
   prepare "bundled gems", gem_dir
   RbInstall.no_write do
+    # Record making directories
     makedirs(Gem.ensure_gem_subdirectories(install_dir, $dir_mode).map {|d| File.join(gem_dir, d)})
   end