[DOC] man: Add environment variable RUBY_PAGER

man/ruby.1

@@ -535,6 +535,14 @@ If set to
 M:N Thread scheduler is enabled on the main Ractor.
 Introduced in Ruby 3.3, default: unset.
 .Pp
+.It Ev RUBY_PAGER
+The pager command that will be used for
+.Fl -help
+option.
+Introduced in Ruby 3.0, default:
+.Ev PAGER
+environment variable.
+.Pp
 .It Ev PATH
 Ruby refers to the
 .Ev PATH