Extract api docs from plugin controller sub-folders

closes LS-3727
flag = none

Test plan:
 - In a plugin, create a file like app/controllers/subfolder/test_controller.rb
 - In that file, add an action with API documentation (see example below)
 - Run `rake doc:api`
 - Ensure that the new documentation appears in <canvas>/doc/api

Example action:
  # @API Test documentation
  # Testing 123!
  class TestController < ApplicationController
  end

Change-Id: I39b238645663f71ef46123e4f1f014b7665f31bd
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/309127
Reviewed-by: Luis Oliveira <luis.oliveira@instructure.com>
QA-Review: Luis Oliveira <luis.oliveira@instructure.com>
Product-Review: Jackson Howe <jackson.howe@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

lib/tasks/docs.rake

@@ -20,7 +20,7 @@ begin
       t.before = proc { `script/generate_lti_variable_substitution_markdown` }
       t.files = %w[
         app/controllers/**/*.rb
-        {gems,vendor}/plugins/*/app/controllers/*.rb
+        {gems,vendor}/plugins/*/app/controllers/**/*.rb
         {gems,vendor}/plugins/*/lib/*.rb
       ]