[ruby/irb] Require RDoc in `input-method.rb` again in a limited scope.

RDoc is implemented as soft dependency in IRB. See how the rdoc is required in
the files. I reverted the commit below.

```
$ grep -ril rdoc lib/
lib/irb/cmd/help.rb
lib/irb/completion.rb
lib/irb/easter-egg.rb
lib/irb/input-method.rb
```

---

Revert "Remove `require` in signal handler to avoid ThreadError"

This reverts commit https://github.com/ruby/irb/commit/5f749c613c89.

https://github.com/ruby/irb/commit/b24852058f

lib/irb/input-method.rb

@@ -320,6 +320,11 @@ module IRB
         [195, 164], # The "ä" that appears when Alt+d is pressed on xterm.
         [226, 136, 130] # The "∂" that appears when Alt+d in pressed on iTerm2.
       ]
+      begin
+        require 'rdoc'
+      rescue LoadError
+        return nil
+      end
 
       if just_cursor_moving and completion_journey_data.nil?
         return nil