[ruby/irb] Add show_doc as an alias to the help command

(https://github.com/ruby/irb/pull/475)

In the long-term, we want to align with `Pry`, `byebug` and `debug` to
use the `help` command to list all commands, which is what `show_cmds`
currently does. And `show_doc` will be the command to look up Ruby APIs.

By aliasing `show_doc` to the current `help` now, users will have time
to get use to it.

lib/irb/extend-command.rb

@@ -162,6 +162,7 @@ module IRB # :nodoc:
 
       [
         :irb_help, :Help, "cmd/help",
+        [:show_doc, NO_OVERRIDE],
         [:help, NO_OVERRIDE],
       ],
 

test/irb/test_cmd.rb

@@ -376,16 +376,18 @@ module TestIRB
       assert_match(/Please specify the file name./, out)
     end
 
-    def test_help
+    def test_help_and_show_doc
+      ["help", "show_doc"].each do |cmd|
         out, _ = execute_lines(
-        "help 'String#gsub'\n",
+          "#{cmd} 'String#gsub'\n",
           "\n",
         )
 
         # the former is what we'd get without document content installed, like on CI
         # the latter is what we may get locally
         possible_rdoc_output = [/Nothing known about String#gsub/, /Returns a copy of self with all occurrences of the given pattern/]
-      assert(possible_rdoc_output.any? { |output| output.match?(out) }, "Expect the help command to match one of the possible outputs")
+        assert(possible_rdoc_output.any? { |output| output.match?(out) }, "Expect the `#{cmd}` command to match one of the possible outputs")
+      end
     ensure
       # this is the only way to reset the redefined method without coupling the test with its implementation
       EnvUtil.suppress_warning { load "irb/cmd/help.rb" }